kunit: respect KBUILD_OUTPUT env variable by default

kunit: respect KBUILD_OUTPUT env variable by default

Currently, kunit.py ignores the KBUILD_OUTPUT env variable and always
defaults to .kunit in the working directory. This behavior is inconsistent
with standard Kbuild behavior, where KBUILD_OUTPUT defines the build
artifact location.

This patch modifies kunit.py to respect KBUILD_OUTPUT if set.  A .kunit
subdirectory is created inside KBUILD_OUTPUT to avoid polluting the build
directory.

Link: https://lore.kernel.org/r/20260106-kunit-kbuild_output-v2-1-582281797343@gmail.com
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Ryota Sakamoto <sakamo.ryota@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>