Some nightly builds occasionally leave some log files behind due to
local configuration. Remove them before checking the source tree since
they are known to not be CMake bugs leaving behind source tree
modifications in the test suite.
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.
