[SR-6792] dispatch_group test is timing out on a fairly regular basis

is duplicated by:

• SR-8664 dispatch_group test timed out on ci.swift.org Ubuntu 14.04

Issue Description:

https://ci.swift.org/job/oss-swift-4.1-incremental-RA-linux-ubuntu-16_10/753/

Test project /home/buildnode/jenkins/workspace/oss-swift-4.1-incremental-RA-linux-ubuntu-16_10/buildbot_incremental/libdispatch-linux-x86_64 Start 1: dispatch_apply 1/21 
Test #​1: dispatch_apply ................... Passed 0.25 sec Start 2: dispatch_api 2/21 
Test #​2: dispatch_api ..................... Passed 0.17 sec Start 3: dispatch_debug 3/21 
Test #​3: dispatch_debug ................... Passed 0.15 sec Start 4: dispatch_queue_finalizer 4/21 
Test #​4: dispatch_queue_finalizer ......... Passed 1.18 sec Start 5: dispatch_group 5/21 
Test #​5: dispatch_group ...................***Timeout 30.07 sec Start 6: dispatch_overcommit 6/21 
Test #​6: dispatch_overcommit .............. Passed 0.68 sec Start 7: dispatch_context_for_key 7/21 
Test #​7: dispatch_context_for_key ......... Passed 0.20 sec Start 8: dispatch_after 8/21 
Test #​8: dispatch_after ................... Passed 9.17 sec Start 9: dispatch_timer 9/21 
Test #​9: dispatch_timer ................... Passed 2.16 sec Start 10: dispatch_timer_short 10/21 Test #​10: dispatch_timer_short ............. Passed 1.44 sec Start 11: dispatch_timer_timeout 11/21 
Test #​11: dispatch_timer_timeout ........... Passed 6.17 sec Start 12: dispatch_sema 12/21 Test #​12: dispatch_sema .................... Passed 0.18 sec Start 13: dispatch_timer_bit31 13/21 
Test #​13: dispatch_timer_bit31 ............. Passed 2.32 sec Start 14: dispatch_timer_bit63 14/21 
Test #​14: dispatch_timer_bit63 ............. Passed 1.16 sec Start 15: dispatch_timer_set_time 15/21 
Test #​15: dispatch_timer_set_time .......... Passed 2.47 sec Start 16: dispatch_starfish 16/21 Test #​16: dispatch_starfish ................ Passed 7.83 sec Start 17: dispatch_data 17/21 
Test #​17: dispatch_data .................... Passed 0.16 sec Start 18: dispatch_io_net 18/21 
Test #​18: dispatch_io_net .................. Passed 0.43 sec Start 19: dispatch_select 19/21 
Test #​19: dispatch_select .................. Passed 0.19 sec Start 20: dispatch_c99 20/21 
Test #​20: dispatch_c99 ..................... Passed 0.19 sec Start 21: dispatch_plusplus 21/21 Test #​21: dispatch_plusplus ................ Passed 0.19 sec

Review the dispatch_group test in the libdispatch test suite and the linked CI job output, starting with the repeated timeout at test 5/21. Check the superseding SR-8664 issue before beginning work. Done means the dispatch_group test completes reliably without timing out in the affected CI run.