Summary:
Disable `TimerTest.SingleScheduleRepeatedlyTest` and `TimerTest.MultipleScheduleRepeatedlyTest`. This is to help people to not hit any hangs (https://github.com/facebook/rocksdb/issues/6698) during their development process while I investigate further; I could not reproduce the issue on my dev machine yet. Note that timer is not being utilized anywhere yet.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6833
Test Plan:
```
svemuri@devbig187 ~/rocksdb (timer-disable-test) $ TEST_TMPDIR=/dev/shm ./timer_test
[==========] Running 2 tests from 1 test case.
[----------] Global test environment set-up.
[----------] 2 tests from TimerTest
[ RUN ] TimerTest.SingleScheduleOnceTest
[ OK ] TimerTest.SingleScheduleOnceTest (1 ms)
[ RUN ] TimerTest.MultipleScheduleOnceTest
[ OK ] TimerTest.MultipleScheduleOnceTest (0 ms)
[----------] 2 tests from TimerTest (1 ms total)
[----------] Global test environment tear-down
[==========] 2 tests from 1 test case ran. (1 ms total)
[ PASSED ] 2 tests.
YOU HAVE 2 DISABLED TESTS
```
Reviewed By: pdillinger
Differential Revision: D21502474
Pulled By: sagar0
fbshipit-source-id: ac67caee2011fd14ffb2476a8914a6286a4f9abe
2e9324718a