Testsuite: run each test in its own /tmp directory, after copying required files

Plan

For each TEST:

• create a directory <testdir> inside /tmp
• link/copy all source files that the test needs into <testdir>
• run the test inside <testdir>
• delete <testdir>

Benefits

• running tests in parallel always works, even when two tests write to the same file (for example .hi/.o files, from sharing a .hs file). This should reduce the number of unsuspected and annoying validate failures. There is no longer any need to specify -outputdir explicitly.
• there is no longer any need to add entries to the perpetually out-of-date testsuite/.gitignore.
• there is no longer any need to specify extra_clean or clean_cmd setup functions for tests. Sometimes you //will// have to specify which extra files a test needs, using the (new) extra_files setup function, but it will be immediately clear when to do so (i.e. the test won't run if you don't). By default only files with the name <testname>* are copied to <testdir>.
• it could become possible to run //ways// for a single test in parallel (i.e. make TEST=<testname> slow would become faster)
• cleaning the testsuite won't be necessary (except for a few files in testsuite/mk perhaps)

Currently the only way to clean the testsuite is via make CLEANUP=1 CLEAN_ONLY=1, which requires a call to the testsuite, which requires building mk/ghc-config first. This will all become much simpler.