At present we normally write tests either in Python or in C. But most
Python tests end up doing a lot of checks which would be better done in C.
Checks done in C are orders of magnitude faster and it is possible to get
full access to U-Boot's internal workings, rather than just relying on
the command line.

The model is to have a Python test set up some things and then use C code
(in a unit test) to check that they were done correctly. But we don't want
those checks to happen as part of normal test running, since each C unit
tests is dependent on the associate Python tests, so cannot run without
it.

To acheive this, add a new UT_TESTF_MANUAL flag to use with the C 'check'
tests, so that they can be skipped by default when the 'ut' command is
used. Require that tests have a name ending with '_norun', so that pytest
knows to skip them.

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

Add a second on writing tests, covering when to use Python and C, where
to put the tests, etc. Add a link to the existing Python test
documentation.

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