selftests/harness: Limit step counter reporting

When the selftest "step" counter grew beyond 255, non-fatal warnings
were being emitted, which is noisy and pointless. There are selftests
with more than 255 steps (especially those in loops, etc). Instead,
just cap "steps" to 254 and do not report the saturation.

Reported-by: Ralph Campbell <rcampbell@nvidia.com>
Tested-by: Ralph Campbell <rcampbell@nvidia.com>
Fixes: 9847d24a ("selftests/harness: Refactor XFAIL into SKIP")
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>