doc: pytest: Document the test_bootstage test

doc: pytest: Document the test_bootstage test

Add this test to the documentation. We need to move the import to follow
the main comment so that it renders correctly, and add a code-block
annotation to the example and indent it correctly. Next, neither of the
functions had comments themselves, so document them now.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>