selftests/bpf: Skip tests whose objects were not built

selftests/bpf: Skip tests whose objects were not built

When both run_test and run_serial_test are NULL (because the corresponding
.test.o was not compiled), mark the test as not built instead of fatally
aborting.

Report these tests as "SKIP (not built)" in per-test output and include
them in the skip count so they remain visible in CI results and JSON
output. The summary line shows the not-built count when nonzero:

  Summary: 50/55 PASSED, 5 SKIPPED (3 not built), 0 FAILED

Tests filtered out by -t/-n remain invisible as before; only genuinely
unbuilt tests are surfaced.

Signed-off-by: Ricardo B. Marlière <rbm@suse.com>
Link: https://lore.kernel.org/r/20260602-selftests-bpf_misconfig-v12-7-27f898b3ba26@suse.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>