kunit: Kconfig: enable a KUNIT_ALL_TESTS fragment

Make it easier to enable all KUnit fragments.  This is useful for kernel
devs or testers, so its easy to get all KUnit tests enabled and if new
gets added they will be enabled as well.  Fragments that has to be
builtin will be missed if CONFIG_KUNIT_ALL_TESTS is set as a module.

Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>