Restore TestKit multi-version compatibility tests
Restore TestKit multi-version compatibility tests

They were not running for a long while ...

The :testKit:crossVersionTests task was running tests from the `test`
source set instead of from the `integTest` source set.

Prefer using `integMultiVersionTest` as those compatibility tests aren't
"cross version tests" per se.

This requires disabling the JUnit category filter set by the
IntegrationTestsPlugin because the TestKit compatibility tests are not
using `@ContextualMultiVersionTest` but a dedicated runner.

Signed-off-by: Paul Merlin <a027184a55211cd23e3f3094f1fdc728df5e0500@gradle.com>

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