Merge patch series "test: Refactor FIT test for clarity and extensibility"

Simon Glass <sjg@chromium.org> says:

This series reworks the FIT test (test_fit.py) to make it easier to
maintain and extend. It fixes the buildconfigspec so the test actually
runs on sandbox, avoids unnecessary U-Boot restarts, renames the main
test for easier selection, adds a missing-kernel check, fixes Python
warnings, converts the test to use a class, splits into separate test
functions, and adds Sphinx documentation.

Link: https://lore.kernel.org/r/20260413130007.1987326-1-sjg@chromium.org