veristat: add ability to filter top N results

veristat: add ability to filter top N results

Add ability to filter top B results, both in replay/verifier mode and
comparison mode. Just adding `-n10` will emit only first 10 rows, or
less, if there is not enough rows.

This is not just a shortcut instead of passing veristat output through
`head`, though. Filtering out all the other rows influences final table
formatting, as table column widths are calculated based on actual
emitted test.

To demonstrate the difference, compare two "equivalent" forms below, one
using head and another using -n argument.

TOP N FEATURE
=============
[vmuser@archvm bpf]$ sudo ./veristat -C ~/baseline-results-selftests.csv ~/sanity2-results-selftests.csv -e file,prog,insns,states -s '|insns_diff|' -n10
File                                      Program                Insns (A)  Insns (B)  Insns (DIFF)  States (A)  States (B)  States (DIFF)
----------------------------------------  ---------------------  ---------  ---------  ------------  ----------  ----------  -------------
test_seg6_loop.bpf.linked3.o              __add_egr_x                12440      12360  -80 (-0.64%)         364         357    -7 (-1.92%)
async_stack_depth.bpf.linked3.o           async_call_root_check        145        145   +0 (+0.00%)           3           3    +0 (+0.00%)
async_stack_depth.bpf.linked3.o           pseudo_call_check            139        139   +0 (+0.00%)           3           3    +0 (+0.00%)
atomic_bounds.bpf.linked3.o               sub                            7          7   +0 (+0.00%)           0           0    +0 (+0.00%)
bench_local_storage_create.bpf.linked3.o  kmalloc                        5          5   +0 (+0.00%)           0           0    +0 (+0.00%)
bench_local_storage_create.bpf.linked3.o  sched_process_fork            22         22   +0 (+0.00%)           2           2    +0 (+0.00%)
bench_local_storage_create.bpf.linked3.o  socket_post_create            23         23   +0 (+0.00%)           2           2    +0 (+0.00%)
bind4_prog.bpf.linked3.o                  bind_v4_prog                 358        358   +0 (+0.00%)          33          33    +0 (+0.00%)
bind6_prog.bpf.linked3.o                  bind_v6_prog                 429        429   +0 (+0.00%)          37          37    +0 (+0.00%)
bind_perm.bpf.linked3.o                   bind_v4_prog                  15         15   +0 (+0.00%)           1           1    +0 (+0.00%)

PIPING TO HEAD
==============
[vmuser@archvm bpf]$ sudo ./veristat -C ~/baseline-results-selftests.csv ~/sanity2-results-selftests.csv -e file,prog,insns,states -s '|insns_diff|' | head -n12
File                                                   Program                                               Insns (A)  Insns (B)  Insns (DIFF)  States (A)  States (B)  States (DIFF)
-----------------------------------------------------  ----------------------------------------------------  ---------  ---------  ------------  ----------  ----------  -------------
test_seg6_loop.bpf.linked3.o                           __add_egr_x                                               12440      12360  -80 (-0.64%)         364         357    -7 (-1.92%)
async_stack_depth.bpf.linked3.o                        async_call_root_check                                       145        145   +0 (+0.00%)           3           3    +0 (+0.00%)
async_stack_depth.bpf.linked3.o                        pseudo_call_check                                           139        139   +0 (+0.00%)           3           3    +0 (+0.00%)
atomic_bounds.bpf.linked3.o                            sub                                                           7          7   +0 (+0.00%)           0           0    +0 (+0.00%)
bench_local_storage_create.bpf.linked3.o               kmalloc                                                       5          5   +0 (+0.00%)           0           0    +0 (+0.00%)
bench_local_storage_create.bpf.linked3.o               sched_process_fork                                           22         22   +0 (+0.00%)           2           2    +0 (+0.00%)
bench_local_storage_create.bpf.linked3.o               socket_post_create                                           23         23   +0 (+0.00%)           2           2    +0 (+0.00%)
bind4_prog.bpf.linked3.o                               bind_v4_prog                                                358        358   +0 (+0.00%)          33          33    +0 (+0.00%)
bind6_prog.bpf.linked3.o                               bind_v6_prog                                                429        429   +0 (+0.00%)          37          37    +0 (+0.00%)
bind_perm.bpf.linked3.o                                bind_v4_prog                                                 15         15   +0 (+0.00%)           1           1    +0 (+0.00%)

Note all the wasted whitespace in the "PIPING TO HEAD" variant.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20231108051430.1830950-2-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>