Add partest to run Scala's test suite.

It comes with a whitelist of tests currently passing,
so that we can easily test for regressions, at least.
add tests considered bugs, create flag to only run tests that are not classified
Fix usage of whitelist.
blacklist REPL tests (indicated by SessionTest)
add comment on how to activate blacklist filtering in partests
classify a couple of tests
add tests using ReplTest to blacklist
classify two more tests