test: Expand implementation of ut_list_has_dm_tests()

test: Expand implementation of ut_list_has_dm_tests()

This function assumes that all tests in a suite are being run. This
means that it can sometimes call dm_test_restore() when it should not.

The impact of this is that it is not possible, for example, to run
'ut bootstd bootflow_cros' and then check the state of bootstd
afterwards, since all devices are removed and recreated.

Update the function to take account of any selected test, to avoid this
problem.

Add a comment for test_insert while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>