KUnit can provide KTAP reports from test modules via debugfs files, one
per test suite.  Using that source of test results instead of extracting
them from dmesg, where they may be interleaved with other kernel messages,
seems more easy to handle and less error prone.  Switch to it.

If KUnit debugfs support is found not configured then fall back to legacy
processing path.

v3: Try to open KUnit debugfs directory before applying KUnit filters we
    use for test case listing, otherwise those skip-all filters applied
    can break legacy path we may enter on missing KUnit debugfs support
    (detected by Kamil).
v2: Check validity of debugfs argument before calling kunit_get_tests()
    (Kamil),
  - replace multiple openat() + fdopen/fdopendir(), each followed by an
    error check, with less expensive fopen/opendir() of file/dir pathname
    components concatenated to a local buffer, protected from buffer
    overflow or truncation with a single check for enough buffer space
    (Lucas),
  - avoid confusing 'if' statement condition (Lucas).

Signed-off-by: Janusz Krzysztofik <janusz.krzysztofik@linux.intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Kamil Konieczny <kamil.konieczny@linux.intel.com>