Major change to the testsuite driver.

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>`

Extra files are (temporarily) tracked in
`testsuite/driver/extra_files.py`, but can also be specified using the
`extra_files` setup function.

Differential Revision: https://phabricator.haskell.org/D1187

Reviewed by: Rufflewind, bgamari

Trac: #11980