Fix virtual_test_suites() for browser_test ports

In r181420 we started pulling the list of virtual test suites to run
from LayoutTests/VirtualTestSuites (a new file). The browser_test ports
do not pull their tests from that directory, and need a different list
of suites, but I forgot to override the port.virtual_test_suites() call
in their implementations.

The browser_test ports don't currently need or have any virtual test suites,
so this patch implements returning an empty list and adds tests to ensure
that we would catch this regression if it happened again.

We also rework the browser_test_unittest.py tests to eliminate a bunch of duplication and make it clearer that the tests are shared and testing the same behavior.

TBR=thestig@chromium.org
BUG=413986

Review URL: https://codereview.chromium.org/569913002

git-svn-id: svn://svn.chromium.org/blink/trunk@181962 bbb929c8-8fbe-4397-9dbb-9b2b20218538