Backport of #3344 onto release-1.5.

The bot's cherry-pick imported hack/e2e-chainsaw/mariadb/chainsaw-test.yaml
whole, because release-1.5 has no such file: the app suite here is still
BATS, and the Chainsaw port landed with #2826 on main only. That file is
dropped and its widening applied to the gate that actually runs on this
branch instead.

That gate needed the change more than the chainsaw one did. In
hack/e2e-apps/mariadb.bats both tests wait 80s for an endpoint address to
appear, and an address appears only once a replica has cleared its startup
then readiness probe. This commit raises the startup budget to 310s, so an
80s ceiling would fail runs the probe was still willing to wait for --
importing the chainsaw file and leaving the BATS suite alone would have
turned the fix into an e2e failure. Both waits go to 600s, matching the
10m the upstream chainsaw assert uses and for the same reason.

packages/apps/mariadb/Makefile gains the `test:` target. It is absent on
release-1.5 -- it arrived with #3071, which was not backported -- and
hack/helm-unit-tests.sh discovers suites by probing for that target, so
without it the backported tests/startup_probe_test.yaml would never be
executed by `make unit-tests`.

Verified locally: 4 assertions in tests/startup_probe_test.yaml pass.

Assisted-By: Claude <[email protected]>
Signed-off-by: Myasnikov Daniil <[email protected]>
(cherry picked from commit a37715062e5da24e54956f075dd2d6417c4dff2e)