dm: test: Allow test names to leave out the dm_test_ prefix

All driver model tests have a dm_test_ prefix. Ignore it when matching a
test name. This makes it easier to run individual tests, like this:

   ./sandbox/u-boot -d ./sandbox/arch/sandbox/dts/test.dtb \
	-c "ut dm clk_periph"

We can use 'clk_periph' instead of 'dm_test_clk_periph'.

Also print a message if the requested test is not found.

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