Added support for -gravitonactivationtime to unit tests

Summary:
This will help prevent situations like D4451 where unit tests fail after an upgrade.

The usage of the graviton upgrade for this diff is after-the-fact because the rest of the codebase has
not yet been migrated to the next upgrade name/time, and this allows for easy verification that this
patch works as expected (see test plan).

This patch has only been prepared as a new feature for the CMake build only as we're beginning to
put Autotools on maintenance-only.

Depends on D4462 and D4496

Test Plan:

cmake -GNinja -DTEST_UPGRADE=On ..
ninja check  # passes

arc patch --skip-dependencies D4462
arc patch --skip-dependencies D4474  # this patch
git checkout 98d6c53  # the commit before D4451
git cherry-pick arcpatch-D4462
git cherry-pick arcpatch-D4474
ninja check  # fails
<build>/src/test/test_bitcoin -gravitonactivationtime=1585000000  # a time in the future so graviton is disabled; passes

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D4474