kunit: tool: print failed tests only

kunit: tool: print failed tests only

Add flag --failed to kunit.py to print only failed tests. This printing
is done after running is over.

This patch also adds the method print_test() that will also print your
Test object. Before, all printing of tests occurred during parsing. This
method could be useful in the future when converting to/from KTAP to this
pretty-print output.

Link: https://lore.kernel.org/r/20241113222406.1590372-2-rmoar@google.com
Signed-off-by: Rae Moar <rmoar@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>