Declare only a single 'BUILD' target for tests

This speeds things up dramatically, especially on Windows, because it
needs to scan the builds only once, rather than for each test binary.

The `alltests` target was added after the BUILD_TEST macro, and defines
which tests we should actually build.

Change-Id: Ibc03373475ed9e73c4df34f70212a65ea8660e1d
Reviewed-on: http://review.couchbase.org/43696
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>