test: Move some test drivers into their own file

test: Move some test drivers into their own file

At present several test drivers are part of the test file itself. Some of
these are useful for of-platdata tests. Separate them out so we can use
them for other things also.

A few adjustments are needed so this driver can build for sandbox_spl as
well.

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