[build-script] Add --parallel to supress output on success

It's slightly faster and it helps supress the test output when tests
succeed (which avoids false positives in Jenkins' error regexes, for
example the error from "let pack" in a test).

Author: benlangmuir

Utilities/build-script-helper.py

@@ -133,7 +133,7 @@ def add_common_args(parser):
     tests = os.path.join(bin_path, 'sk-tests')
     print('Cleaning ' + tests)
     shutil.rmtree(tests, ignore_errors=True)
-    swiftpm('test', swift_exec, swiftpm_args, env)
+    swiftpm('test', swift_exec, swiftpm_args + ['--parallel'], env)
   elif args.action == 'install':
     bin_path = swiftpm_bin_path(swift_exec, swiftpm_args, env)
     swiftpm('build', swift_exec, swiftpm_args, env)