remove dependency on scalatest

currently, finatra will pull in scalatest in an outdated version, which
will cause version conflicts with upstream projects that want to use
newer versions. the only reason for this seems to be the FlatSpecHelper,
which is a one-liner only mixing in the Helper trait. Pull request #59
was already supposed to "Let people choose their test framework" but
didn't go all the way.

this limits the scalatest dependency to the test scope and removes the
FlatSpecHelper from the `.jar`, so it should be considered a breaking
change. I can provide an accompanying pull request to update the
documentation in capotej/finatra-site, but want to see if the maintainers
agree with this change first