lib/cmdline_kunit: add a new test suite for cmdline API

Test get_option() for a starter which is provided by cmdline.c.

[akpm@linux-foundation.org: fix warning by constifying cmdline_test_values]
[andriy.shevchenko@linux.intel.com: type of expected returned values should be int]
  Link: https://lkml.kernel.org/r/20201116104244.15472-1-andriy.shevchenko@linux.intel.com
[andriy.shevchenko@linux.intel.com: provide meaningful MODULE_LICENSE()]
  Link: https://lkml.kernel.org/r/20201116104257.15527-1-andriy.shevchenko@linux.intel.com

Link: https://lkml.kernel.org/r/20201112180732.75589-6-andriy.shevchenko@linux.intel.com


Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Cc: Vitor Massaru Iha <vitor@massaru.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Brendan Higgins <brendanhiggins@google.com>
Cc: David Gow <davidgow@google.com>
Cc: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>