selftests: drv-net: add helper/wrapper for bpftrace

selftests: drv-net: add helper/wrapper for bpftrace

bpftrace is very useful for low level driver testing. perf or trace-cmd
would also do for collecting data from tracepoints, but they require
much more post-processing.

Add a wrapper for running bpftrace and sanitizing its output.
bpftrace has JSON output, which is great, but it prints loose objects
and in a slightly inconvenient format. We have to read the objects
line by line, and while at it return them indexed by the map name.

Reviewed-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20250714-netpoll_test-v7-1-c0220cfaa63e@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>