Update the change from commit 8d453ee751 (Tests: Improve CheckSourceTree
test, 2022-10-05) to drive the test script using the tested CMake
instead of the host CMake, following the convention of other tests.
Re-implement the test using simpler approach. Enable it only when doing
an out-of-source build with a `.git` source. Run it last, serially.
Re-use the `git` tool found for version computation. Print the output
from `git status` without modification. Rely on `.gitignore` instead
of filtering out paths ourselves.