Testsuite: simplify extra_file handling

Authored by thomie on Jun 21 2016, 11:58 AM.

Description

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.