add debugfs support for displaying kunit test suite results; this is
especially useful for module-loaded tests to allow disentangling of
test result display from other dmesg events.  debugfs support is
provided if CONFIG_KUNIT_DEBUGFS=y.

As well as printk()ing messages, we append them to a per-test log.
Signed-off-by: NAlan Maguire <alan.maguire@oracle.com>
Reviewed-by: NBrendan Higgins <brendanhiggins@google.com>
Reviewed-by: NFrank Rowand <frank.rowand@sony.com>
Signed-off-by: NShuah Khan <skhan@linuxfoundation.org>