Investigate C++ flake in the old 0.14 branch

This fixes Jenkins for that branch, as a backwards-incompatible change
was made in the script that runs the tests for all branches.

The lines added are exactly the same as those now in master, so that the
subsequent merge of 0.14 into master doesn’t conflict.

tools/run_tests/run_tests.py

@@ -824,6 +824,8 @@ argp.add_argument('--update_submodules', default=[], nargs='*',
 argp.add_argument('-a', '--antagonists', default=0, type=int)
 argp.add_argument('-x', '--xml_report', default=None, type=str,
         help='Generates a JUnit-compatible XML report')
+argp.add_argument('--force_default_poller', default=False, action='store_const', const=True,
+                  help='Dont try to iterate over many polling strategies when they exist')
 args = argp.parse_args()
 
 jobset.measure_cpu_costs = args.measure_cpu_costs