Make the default .kunitconfig (specified in
arch/um/configs/kunit_defconfig) specify CONFIG_KUNIT_ALL_TESTS by
default. KUNIT_ALL_TESTS runs all tests which have satisfied
dependencies in the current .config (which would be the architecture
defconfig).

Currently, the default .kunitconfig enables only the example tests and
KUnit's own tests. While this does provide a good example of what a
.kunitconfig for running a few individual tests should look like, it
does mean that kunit_tool runs a pretty paltry collection of tests by
default.

The example tests' config entry (CONFIG_KUNIT_EXAMPLE_TEST=y) continues
to be included -- despite now being redundant -- to provide an example
of how tests are enabled when KUNIT_ALL_TESTS is disabled.

A default run of ./tools/testing/kunit/kunit.py run now runs 70 tests
instead of 14.
Signed-off-by: David Gow <davidgow@google.com>
Acked-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>