selftests/bpf: fix test_loader check message

selftests/bpf: fix test_loader check message

Seeing:

  process_subtest:PASS:Can't alloc specs array 0 nsec

... in verbose successful test log is very confusing. Use smaller
identifier-like test tag to denote that we are asserting specs array
allocation success.

Now it's much less distracting:

  process_subtest:PASS:specs_alloc 0 nsec

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20240105000909.2818934-2-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>