meson.eclass: Allow packages to override PLATFORM2_TEST_DEPS

Change PLATFORM2_TEST_DEPS to use a weak assignment so that
ebuilds inheriting meson.eclass can override it.

By default, meson.eclass sets PLATFORM2_TEST_DEPS="build+test",
which forces packages to unconditionally depend on dev-lang/python
and other test dependencies in their DEPEND (target) even when
building without tests (-test).

Allowing packages to override this to "test-only" enables pruning
unnecessary Python runtime/target dependencies for minimal builds
(like firmware).

BUG=None
TEST=emerge-fatcat sys-boot/chromeos-bootimage shows pruned depgraph.
TAG=agy
CONV=29df4e09-3465-47e9-9490-f4d84cf9d049

Change-Id: I033ad8a3837e52e2692faa8a70081df901531118
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/eclass-overlay/+/7981617
Commit-Queue: Raul Rangel <rrangel@chromium.org>
Auto-Submit: Raul Rangel <rrangel@chromium.org>
Tested-by: Raul Rangel <rrangel@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>