selftests/bpf: no need to track next_match_pos in struct test_loader

selftests/bpf: no need to track next_match_pos in struct test_loader

The call stack for validate_case() function looks as follows:
- test_loader__run_subtests()
  - process_subtest()
    - run_subtest()
      - prepare_case(), which does 'tester->next_match_pos = 0';
      - validate_case(), which increments tester->next_match_pos.

Hence, each subtest is run with next_match_pos freshly set to zero.
Meaning that there is no need to persist this variable in the
struct test_loader, use local variable instead.

Acked-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20240722233844.1406874-7-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>