kunit: Print test statistics on failure

When a number of tests fail, it can be useful to get higher-level
statistics of how many tests are failing (or how many parameters are
failing in parameterised tests), and in what cases or suites. This is
already done by some non-KUnit tests, so add support for automatically
generating these for KUnit tests.

This change adds a 'kunit.stats_enabled' switch which has three values:
- 0: No stats are printed (current behaviour)
- 1: Stats are printed only for tests/suites with more than one
     subtest (new default)
- 2: Always print test statistics

For parameterised tests, the summary line looks as follows:
"    # inode_test_xtimestamp_decoding: pass:16 fail:0 skip:0 total:16"
For test suites, there are two lines looking like this:
"# ext4_inode_test: pass:1 fail:0 skip:0 total:1"
"# Totals: pass:16 fail:0 skip:0 total:16"

The first line gives the number of direct subtests, the second "Totals"
line is the accumulated sum of all tests and test parameters.

This format is based on the one used by kselftest[1].

[1]: https://elixir.bootlin.com/linux/latest/source/tools/testing/selftests/kselftest.h#L109

Signed-off-by: David Gow <davidgow@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>