gitaly: Fix transaction manager test flake

Currently the propose returns if context is canceled before admission subtest in TestTransactionManager is flaky due to a race between context cancellation and tranaction admission into the manager. To resolve this, SkipStartManager is added to the step configuration to run the subtest without starting the transaction manager. This prevents transactions from being admitted to the manager and requires the context to be cancelled before the test continues. With the race condtion resolved the quaratine is remove from the test.