Ensure that iOS test re-runs don't try to spawn a process. (python#12…

…2994)

Adds the --single-process option to the iOS test runner to ensure re-runs execute in the same process.

iOS/testbed/iOSTestbedTests/iOSTestbedTests.m

@@ -15,6 +15,7 @@ - (void)testPython {
     const char *argv[] = {
         "iOSTestbed", // argv[0] is the process that is running.
         "-uall",  // Enable all resources
+        "--single-process",  // always run all tests sequentially in a single process
         "--rerun",  // Re-run failed tests in verbose mode
         "-W",  // Display test output on failure
         // To run a subset of tests, add the test names below; e.g.,