selftests: Warn about skipped tests in result summary

Update the functions that print the test totals at the end of a selftest
to include a warning message when skipped tests are detected. The
message advises users that skipped tests may indicate missing
configuration options and suggests enabling them to improve coverage.

Link: https://lore.kernel.org/r/20241126093710.13314-1-laura.nao@collabora.com


Signed-off-by: Laura Nao <laura.nao@collabora.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>