kunit: capture stderr on all make subprocess calls

Direct stderr to subprocess.STDOUT so error messages get included in the
subprocess.CalledProcessError exceptions output field. This results in
more meaningful error messages for the user.

This is already being done in the make_allyesconfig method. Do the same
for make_mrproper, make_olddefconfig, and make methods.

With this, failures on unclean trees [1] will give users an error
message that includes:
"The source tree is not clean, please run 'make ARCH=um mrproper'"

[1] https://bugzilla.kernel.org/show_bug.cgi?id=205219



Signed-off-by: Will Chen <chenwi@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Tested-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>