CppTools: Tests: Stop writing into source dir

Instead, always write into a unique temporary directory in
QDir::tempPath().

Where applicable, read the test source from files instead of first
writing the file.

Some clean ups in test_codegen*.

Change-Id: Id48dc50c6ca16252edfd9fc8a86ba0de9f9be486
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>
Reviewed-by: Erik Verbruggen <erik.verbruggen@theqtcompany.com>