Fixes test flakiness in a new multiversion test that was introduced via SERVER-138293 and caused BF-47119.

The test starts a sharded cluster on mixed versions, which led to the old versions starting up with an unexpected 8.0 FCV. This made the automatic FCV upgrade race with the race of the test, so it could fail on slower build variants.

This change also refactors the test so it reuses the same ShardingTest fixture across multiple test cases, so that overall test runtime is greatly reduced.