kunit: Move default config from arch/um -> tools/testing/kunit

kunit: Move default config from arch/um -> tools/testing/kunit

The default .kunitconfig file is currently kept in
arch/um/configs/kunit_defconfig, but -- with the impending QEMU patch
-- will no-longer be exclusively used for UML-based kernels.

Move it alongside the other KUnit configs in
tools/testing/kunit/configs, and give it a name which matches the
existing all_tests.config and broken_on_uml.config files.

Also update the Getting Started documentation to point to the new file.

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