Testsuite: simplify extra_file handling
Before, `extra_files(['.hpc/Main.mix'])` meant copy `Main.mix` to `<testdir>/.hpc/Main.mix`. This feature wasn't really necessary, so now it just means copy `Main.mix` to `<testdir>/Main.mix`. This simplifies the implementation. Some small other cleanups as well. -40 lines of code.
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