Context:
1. all_tests_uml.config used to be UML specific back when users to
   manually specify CONFIG_VIRTIO_UML=y to enable CONFIG_PCI=y.
2. --alltests used allyesconfig along with a curated list of options to
   disable. It's only ever worked for brief periods of time and has
   perennially been broken due to compile issues.

Now all_tests_uml.config should work across ~all architectures.
Let's instead use this to implement --alltests.

Note: if anyone was using all_tests_uml.config, this change breaks them.
I think that's unlikely since it was added in 5.19 and was a lot to
type: --kunitconfig=tools/testing/kunit/configs/all_tests_uml.config.
We could make it a symlink to the new name, but I don't think the
caution is warranted here.
Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>