kunit: Add --list_suites to show suites

kunit: Add --list_suites to show suites

Currently, kunit.py allows listing all individual tests via --list_tests.
However, users often need to see only the available test suites.

Add --list_suites to show suites. This option parses the test list output
from the kernel and prints only the suite names.

Example of the output of --list_suites:
  example_init
  miscdev_init
  printk-ringbuffer

Signed-off-by: Ryota Sakamoto <sakamo.ryota@gmail.com>
Reviewed-by: David Gow <david@davidgow.net>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>