MB-20054: Backport ep-engine_unit_tests from watson to 3.0.x

MB-20054: Backport ep-engine_unit_tests from watson to 3.0.x

In Watson we have created a set of 'unit' (i.e. class-level) tests for
ep-engine. To assist in backporting bug fixes, and specifically their
unit tests (to demonstrate they are correct), this patch backports the
test infrastructure itself.

Note these tests require GTest, so the CMake changes necessary for it
have also been included.

Tests are a backport from couchbase/watson as of commit feda304.
Modified to handle changes in APIs etc, and to remove tests
which fail on 3.0.x as we never chose to fix them in the 3.0.x
branch.

Change-Id: Iaaf59b0d8d6ba0a2211b630ba00fd837ca01614a
Reviewed-on: http://review.couchbase.org/64979
Well-Formed: buildbot <build@couchbase.com>
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Jim Walker <jim@couchbase.com>