test: Rename all linker lists to have a ut_ prefix

At present each test suite has its own portion of the linker_list section
of the image, but other lists are interspersed. This makes it hard to
enumerate all the available tests without knowing the suites that each one
is in.

Place all tests together in a single contiguous list by giving them
common prefix not used elsewhere in U-Boot. This makes it possible to find
the start and end of all tests.

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