Add --serial mode to dm, runs unit tests serially

infra/bots/recipes/swarm_test.expected/Test-Ubuntu-Clang-GCE-CPU-AVX2-x86_64-Coverage-Trybot.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "\nimport shutil\nimport sys\nshutil.copy(sys.argv[1], sys.argv[2])\n",
       "[SLAVE_BUILD]/skia/infra/bots/assets/skp/VERSION",
       "/path/to/tmp/"
     ],
     "name": "Get downloaded SKP VERSION"
@@ skipping 417 matching lines @@
       "_",
       "Seal_of_Kansas.svg",
       "2ndpic-8888",
       "svg",
       "_",
       "Seal_of_Kansas.svg",
       "serialize-8888",
       "svg",
       "_",
       "Seal_of_Kansas.svg",
+      "--match",
+      "~ColorSpaceCRBug",
+      "--solo",
+      "ColorSpaceCRBug",
       "--outResultsFile",
       "[SLAVE_BUILD]/out/coverage_results/abc123.cov"
     ],
     "cwd": "[SLAVE_BUILD]/skia",
     "env": {
       "BUILDTYPE": "Coverage",
       "CHROME_HEADLESS": "1",
       "SKIA_OUT": "[SLAVE_BUILD]/out"
     },
     "name": "dm"
@@ skipping 87 matching lines @@
       "[CUSTOM_[SWARM_OUT_DIR]]"
     ],
     "name": "Copy to swarming out (2)"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]