Add TestProject wrapper for mutable tests. Change-Id: I1e03bc8fbb78411375777388620f522cde12b21f Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/qg/+/126771 Commit-Queue: Erik Gilling <konkers@google.com> Reviewed-by: Alexei Frolov <frolv@google.com>