Summary:
I noticed tests frequently timing out on CircleCI when I submit a PR.  I did some investigation and found the SeqAdvanceConcurrentTest suite (OneWriteQueue, TwoWriteQueues) tests were all taking a long time to complete (30 tests each taking at least 15K ms).

This PR adds those test to the "slow reg" list in order to move them earlier in the execution sequence so that they are not the "long tail".

For completeness, other tests that were also slow are:
NumLevels/DBTestUniversalCompaction.UniversalCompactionTrivialMoveTest : 12 tests all taking 12K+ ms
ReadSequentialFileTest with ReadaheadSize: 8 tests all 12K+ ms
WriteUnpreparedTransactionTest.RecoveryTest : 2 tests at 22K+ ms
DBBasicTest.EmptyFlush: 1 test at 35K+ ms
RateLimiterTest.Rate: 1 test at 23K+ ms
BackupableDBTest.ShareTableFilesWithChecksumsTransition: 1 test at 16K+ ms
MulitThreadedDBTest.MultitThreaded: 78 tests at 10K+ ms
TransactionStressTest.DeadlockStress: 7 tests at 11K+ ms
DBBasicTestDeadline.IteratorDeadline: 3 tests at 10K+ ms

No effort was made to determine why the tests were slow.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7973

Reviewed By: jay-zhuang

Differential Revision: D26519130

Pulled By: mrambacher

fbshipit-source-id: 11555c9115acc207e45e210a7fc7f879170a3853