[Backport 6.7] Ensure global test seed is used for all random testing tasks (#38991)

[mark-vieira]

This commit fixes a bug which resulted in every RandomizedTestingTask
generating its own unique test seed rather than using the global test
seed generated by the the RandomizedTestingPlugin.

The issue didn't affect build invocations which explicitly ran with
-Dtest.seed. In those cases, the Ant junit task correctly uses the
global seed.

Since the Ant random testing target looks for an Ant project property
for determining the existing seed we now explicitly set this inside
RandomizedTestingPlugin. It just so happens that, incidentally, Ant will
inherit system properties, thus wy the -Dtest.seed mechanism continued
to work.

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