Run 2 parallel jobs for browser_tests on Mac10.13 dbg

The job usually get killed after ~80% of all tests. So 2 launcherjobs
should get all tests work.

Bug: 1164430, 11527703
Change-Id: Ib582a92aafd23cb261561cc772d0842dd63804e5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2622424
Auto-Submit: Sven Zheng <svenzheng@chromium.org>
Commit-Queue: Stephen Martinis <martiniss@chromium.org>
Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842212}

testing/buildbot/chromium.mac.json

@@ -5654,7 +5654,7 @@
       },
       {
         "args": [
-          "--test-launcher-jobs=1"
+          "--test-launcher-jobs=2"
         ],
         "merge": {
           "args": [],

testing/buildbot/test_suite_exceptions.pyl

@@ -523,7 +523,7 @@
       'Mac10.13 Tests (dbg)': {
         # https://crbug.com/1152770
         'args': [
-          '--test-launcher-jobs=1',
+          '--test-launcher-jobs=2',
         ],
         'swarming': {
           'shards': 20,