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.