test/py/tests/test_bootstage.py: Combine stash/unstash tests

When running the bootstage tests currently we get a warning like:
tests/test_bootstage.py::test_bootstage_stash
...PytestReturnNotNoneWarning: Expected None, but tests/test_bootstage.py::test_bootstage_stash returned (37748736, 4096), which will be an error in a future version of pytest.  Did you mean to use `assert` in stead of `return`?

This is because the unstash test will run the stash test and fetch the
addr / size from that. Rework the test to be stash and unstash and then
run the unstash command at the end of the current stash test.

Signed-off-by: Tom Rini <trini@konsulko.com>
---
It's possible we could solve this instead by getting addr / size again
from our environment file, and then also noting that order-wise the
unstash test follows the stash test. However, this felt more clean to
me.

Cc: Love Kumar <love.kumar@amd.com>
Cc: Michal Simek <michal.simek@amd.com>
Cc: Simon Glass <sjg@chromium.org>