selftests/harness: Handle TEST_F()'s explicit exit codes

selftests/harness: Handle TEST_F()'s explicit exit codes

If TEST_F() explicitly calls exit(code) with code different than 0, then
_metadata->exit_code is set to this code (e.g. KVM_ONE_VCPU_TEST()).  We
need to keep in mind that _metadata->exit_code can be KSFT_SKIP while
the process exit code is 0.

Cc: Jakub Kicinski <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Mark Brown <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Will Drewry <[email protected]>
Reported-by: Sean Christopherson <[email protected]>
Tested-by: Sean Christopherson <[email protected]>
Closes: https://lore.kernel.org/r/[email protected]
Fixes: 0710a1a73fb4 ("selftests/harness: Merge TEST_F_FORK() into TEST_F()")
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Mickaël Salaün <[email protected]>