Added arcanist (arc) unit test support

  Thanks to Clemens (hammacher@cs.uni-saarland.de) extension of the
  arcanist unit test engine, we can run the llvm-lit tests automatically.

  Similar to the linters, aracanist will run all tests for each uploaded
  commit and asks the commiter for a statements in case unit tests fail.

  To use this feature the unit test engine needs to find the polly build
  directory. Currently we support the following setups:

    $POLLY_BUILD_DIR (environment variable)
    <root>/build
    <root>.build
    <root>-build
    <root:s/src/build>
    <cwd>