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

infra/bots/recipes/swarm_test.expected/Test-Ubuntu-GCC-ShuttleA-GPU-GTX550Ti-x86_64-Release-Valgrind.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 469 matching lines @@
       ".RW2",
       "_",
       "image",
       "_",
       ".PEF",
       "_",
       "image",
       "_",
       ".SRW",
       "--match",
-      "~Threaded"
+      "~Threaded",
+      "--serial",
+      "ColorSpaceCRBug"
     ],
     "env": {
       "BUILDTYPE": "Release",
       "CHROME_HEADLESS": "1",
       "SKIA_OUT": "[SLAVE_BUILD]/out"
     },
     "name": "dm"
   },
   {
     "cmd": [
@@ skipping 417 matching lines @@
       "_",
       "image",
       "_",
       ".PEF",
       "_",
       "image",
       "_",
       ".SRW",
       "--match",
       "~Threaded",
+      "--serial",
+      "ColorSpaceCRBug",
       "--abandonGpuContext"
     ],
     "env": {
       "BUILDTYPE": "Release",
       "CHROME_HEADLESS": "1",
       "SKIA_OUT": "[SLAVE_BUILD]/out"
     },
     "name": "dm --abandonGpuContext"
   },
   {
@@ skipping 418 matching lines @@
       "_",
       "image",
       "_",
       ".PEF",
       "_",
       "image",
       "_",
       ".SRW",
       "--match",
       "~Threaded",
+      "--serial",
+      "ColorSpaceCRBug",
       "--preAbandonGpuContext"
     ],
     "env": {
       "BUILDTYPE": "Release",
       "CHROME_HEADLESS": "1",
       "SKIA_OUT": "[SLAVE_BUILD]/out"
     },
     "name": "dm --preAbandonGpuContext"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]