Add a --reports-dir option to define a reports directory.

In the future, we should start tor using tor-launcher. For now, the
tbb-testsuite script will start tor before running the tests and skip
tor-launcher by setting the TOR_SKIP_LAUNCH environment variable.

If we are given a sha256sums.txt file as argument, then we test all the
bundles compatible with current platform listed in the sha256sums.txt.
If sha256sums.txt file is a URL, then we download it and the bundle
files that are listed in it, and check their sha256sum.