[Dolphin] Refactor recurrence ranker tests.

This makes each ranker test construct its own ranker, instead of using a
member constructed in SetUp. This makes testing UMA error metrics
easier, which will be a follow-up CL.

Bug: 921444
Change-Id: I0dbd2f09796e5a9773318eda2a0533fbdc2d9bfd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1614643
Reviewed-by: Charles . <charleszhao@chromium.org>
Commit-Queue: Tony Yeoman <tby@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660712}
1 file changed