selftests/seccomp: fix negative_ENOSYS tracer tests on arm32

selftests/seccomp: fix negative_ENOSYS tracer tests on arm32

TRACE_syscall.ptrace.negative_ENOSYS and TRACE_syscall.seccomp.negative_ENOSYS
on arm32 are being reported as failures instead of skipping.

The teardown_trace_fixture function sets the test to KSFT_FAIL in case of a
non 0 return value from the tracer process.
Due to _metadata now being shared between the forked processes the tracer is
returning the KSFT_SKIP value set by the tracee which is non 0.

Remove the setting of the _metadata.exit_code in teardown_trace_fixture.

Fixes: 24cf65a62266 ("selftests/harness: Share _metadata between forked processes")
Signed-off-by: Terry Tritton <terry.tritton@linaro.org>
Link: https://lore.kernel.org/r/20250509115622.64775-1-terry.tritton@linaro.org
Signed-off-by: Kees Cook <kees@kernel.org>