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 <[email protected]>
Reviewed-by: Simon Glass <[email protected]>

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