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

infra/bots/recipes/swarm_test.expected/Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-TSAN.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 354 matching lines @@
       "Seal_of_Kansas.svg",
       "2ndpic-8888",
       "svg",
       "_",
       "Seal_of_Kansas.svg",
       "serialize-8888",
       "svg",
       "_",
       "Seal_of_Kansas.svg",
       "--match",
-      "~ReadWriteAlpha"
+      "~ReadWriteAlpha",
+      "--serial",
+      "ColorSpaceCRBug"
     ],
     "env": {
       "ASAN_OPTIONS": "symbolize=1 detect_leaks=1",
       "BUILDTYPE": "Release",
       "CHROME_HEADLESS": "1",
       "LD_LIBRARY_PATH": "[SLAVE_BUILD]/third_party/externals/llvm/msan_out/lib",
       "LSAN_OPTIONS": "symbolize=1 print_suppressions=1 suppressions=[SLAVE_BUILD]/skia/tools/lsan.supp",
       "PATH": "%(PATH)s:[SLAVE_BUILD]/llvm-build/Release+Asserts/bin",
       "SKIA_OUT": "[SLAVE_BUILD]/out",
       "TSAN_OPTIONS": "suppressions=[SLAVE_BUILD]/skia/tools/tsan.supp",
       "UBSAN_OPTIONS": "suppressions=[SLAVE_BUILD]/skia/tools/ubsan.supp"
     },
     "name": "dm"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]