webrtc: use correct cwd for running tests

scripts/slave/recipes/webrtc/standalone.expected/tryserver_webrtc_mac_baremetal.json

 [
   {
     "cmd": [
       "RECIPE_PACKAGE_REPO[build]/scripts/slave/ios/host_info.py",
       "--json-file",
       "/path/to/tmp/json"
     ],
     "name": "host and tools info",
     "~followup_annotations": [
       "@@@STEP_TEXT@<br />OS X 1.2.3, Xcode 6.7.8 (5D342509a)@@@",
@@ skipping 247 matching lines @@
       "@@@STEP_LOG_LINE@json.output (invalid)@null@@@",
       "@@@STEP_LOG_END@json.output (invalid)@@@"
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[build::webrtc]/resources/ensure_webcam_is_running.py"
     ],
+    "cwd": "[BUILDER_CACHE]/mac_baremetal",
     "name": "webcam_check"
   },
   {
     "allow_subannotations": true,
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]/scripts/slave/runtest.py",
       "--target",
       "Release",
       "--factory-properties",
       "{\"BUILD_CONFIG\": \"Release\", \"buildername\": \"mac_baremetal\", \"buildnumber\": 1337, \"issue\": 666666, \"mastername\": \"tryserver.webrtc\", \"patchset\": 1, \"path_config\": \"kitchen\", \"recipe\": \"webrtc/standalone\", \"revision\": \"12345\", \"rietveld\": \"https://fake.rietveld.url\", \"slavename\": \"slavename\"}",
       "--build-properties",
       "{\"BUILD_CONFIG\": \"Release\", \"buildername\": \"mac_baremetal\", \"buildnumber\": 1337, \"issue\": 666666, \"mastername\": \"tryserver.webrtc\", \"patchset\": 1, \"path_config\": \"kitchen\", \"recipe\": \"webrtc/standalone\", \"revision\": \"12345\", \"rietveld\": \"https://fake.rietveld.url\", \"slavename\": \"slavename\"}",
       "--annotate=gtest",
       "--test-type=audio_device_tests",
       "--step-name=audio_device_tests",
       "--builder-name=mac_baremetal",
       "--slave-name=slavename",
       "--build-number=1337",
       "--revision=f27fede2220bcd326aee3e86ddfd4ebd0fe58cb9",
       "audio_device_tests"
     ],
+    "cwd": "[BUILDER_CACHE]/mac_baremetal",
     "name": "audio_device_tests"
   },
   {
     "allow_subannotations": true,
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]/scripts/slave/runtest.py",
       "--target",
       "Release",
       "--factory-properties",
       "{\"BUILD_CONFIG\": \"Release\", \"buildername\": \"mac_baremetal\", \"buildnumber\": 1337, \"issue\": 666666, \"mastername\": \"tryserver.webrtc\", \"patchset\": 1, \"path_config\": \"kitchen\", \"recipe\": \"webrtc/standalone\", \"revision\": \"12345\", \"rietveld\": \"https://fake.rietveld.url\", \"slavename\": \"slavename\"}",
       "--build-properties",
       "{\"BUILD_CONFIG\": \"Release\", \"buildername\": \"mac_baremetal\", \"buildnumber\": 1337, \"issue\": 666666, \"mastername\": \"tryserver.webrtc\", \"patchset\": 1, \"path_config\": \"kitchen\", \"recipe\": \"webrtc/standalone\", \"revision\": \"12345\", \"rietveld\": \"https://fake.rietveld.url\", \"slavename\": \"slavename\"}",
       "--annotate=gtest",
       "--test-type=voe_auto_test",
       "--step-name=voe_auto_test",
       "--builder-name=mac_baremetal",
       "--slave-name=slavename",
       "--build-number=1337",
       "--revision=f27fede2220bcd326aee3e86ddfd4ebd0fe58cb9",
       "voe_auto_test",
       "--automated"
     ],
+    "cwd": "[BUILDER_CACHE]/mac_baremetal",
     "name": "voe_auto_test"
   },
   {
     "allow_subannotations": true,
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]/scripts/slave/runtest.py",
       "--target",
       "Release",
       "--factory-properties",
       "{\"BUILD_CONFIG\": \"Release\", \"buildername\": \"mac_baremetal\", \"buildnumber\": 1337, \"issue\": 666666, \"mastername\": \"tryserver.webrtc\", \"patchset\": 1, \"path_config\": \"kitchen\", \"recipe\": \"webrtc/standalone\", \"revision\": \"12345\", \"rietveld\": \"https://fake.rietveld.url\", \"slavename\": \"slavename\"}",
       "--build-properties",
       "{\"BUILD_CONFIG\": \"Release\", \"buildername\": \"mac_baremetal\", \"buildnumber\": 1337, \"issue\": 666666, \"mastername\": \"tryserver.webrtc\", \"patchset\": 1, \"path_config\": \"kitchen\", \"recipe\": \"webrtc/standalone\", \"revision\": \"12345\", \"rietveld\": \"https://fake.rietveld.url\", \"slavename\": \"slavename\"}",
       "--annotate=gtest",
       "--test-type=video_capture_tests",
       "--step-name=video_capture_tests",
       "--builder-name=mac_baremetal",
       "--slave-name=slavename",
       "--build-number=1337",
       "--revision=f27fede2220bcd326aee3e86ddfd4ebd0fe58cb9",
       "video_capture_tests"
     ],
+    "cwd": "[BUILDER_CACHE]/mac_baremetal",
     "name": "video_capture_tests"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]