Move more unit tests into the ep_unit_tests binary

Move more unit tests into the ep_unit_tests binary

For GTest-style tests we have created a single test binary
(ep-engine_ep_unit_tests) to link all the tests into. This has the
advantage of not having to compile different variants of our source
files for multiple different test binaries (which is partly a
limitation / 'feature' of CMake's dependancy calculation).

However not all tests are in this binary. This patch moves an
additional 2 test suites - checkpoint & defragmenter - into the single
binary. This speeds up our build time, and also removes a bunch of
duplicated boilerplate test setup code.

Change-Id: I7a9b6f8166fe2dcb739bdf124b43d1de6abc1e42
Reviewed-on: http://review.couchbase.org/68212
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Manu Dhundi <manu@couchbase.com>