unit-tests: switch back to running unit-tests sequentially.

The recent patch that upgrades FindBugs to v3 (8bd68c5)
also switches on multi-threaded unit-testing.

Unfortunately, this breaks the unit-tests in 2.6 branch.

At least for now, we revert back to running these tests
one-at-a-time.

Request: 2.6
Acked-by: Tigran Mkrtchyan