Fixes #14097 - Only run DatabaseCleaner truncation on JS tests

Problem:
DatabaseCleaner runs after all integration tests, on truncation mode.
However, for tests where we use rack-test instead of poltergeist, this
is not necessary and slows down the test suite.

Solution:
Mark the tests that use javascript and only run DatabaseCleaner on
truncation mode on these, the rest of them can run in transaction mode
(faster).

Some basic benchmarking of rake test:integration suggests this is 15% faster
than before this patch (before, 3m 42s avg, now 3m 09s avg).

Additionally this patch allows login to happen only once for integration
tests that require JS (it logged in twice before), and loads fewer
dependencies for integration tests when ran on isolation (rake test:integration)