webrtc: use correct cwd for running tests

scripts/slave/recipes/webrtc/standalone.expected/client_webrtc_mac64_release__large_tests_.json

Index: scripts/slave/recipes/webrtc/standalone.expected/client_webrtc_mac64_release__large_tests_.json
diff --git a/scripts/slave/recipes/webrtc/standalone.expected/client_webrtc_mac64_release__large_tests_.json b/scripts/slave/recipes/webrtc/standalone.expected/client_webrtc_mac64_release__large_tests_.json
index 7ae1b9a2e2069c1365205d835d2f751abd56b173..c0f235b9f470da874b4e8b591aeca906a049d79f 100644
--- a/scripts/slave/recipes/webrtc/standalone.expected/client_webrtc_mac64_release__large_tests_.json
+++ b/scripts/slave/recipes/webrtc/standalone.expected/client_webrtc_mac64_release__large_tests_.json
@@ -257,6 +257,7 @@
       "-u",
       "RECIPE_MODULE[build::webrtc]/resources/ensure_webcam_is_running.py"
     ],
+    "cwd": "[BUILDER_CACHE]/Mac64_Release__large_tests_",
     "name": "webcam_check"
   },
   {
@@ -283,6 +284,7 @@
       "--revision=f27fede2220bcd326aee3e86ddfd4ebd0fe58cb9",
       "audio_device_tests"
     ],
+    "cwd": "[BUILDER_CACHE]/Mac64_Release__large_tests_",
     "name": "audio_device_tests"
   },
   {
@@ -310,6 +312,7 @@
       "voe_auto_test",
       "--automated"
     ],
+    "cwd": "[BUILDER_CACHE]/Mac64_Release__large_tests_",
     "name": "voe_auto_test"
   },
   {
@@ -336,6 +339,7 @@
       "--revision=f27fede2220bcd326aee3e86ddfd4ebd0fe58cb9",
       "video_capture_tests"
     ],
+    "cwd": "[BUILDER_CACHE]/Mac64_Release__large_tests_",
     "name": "video_capture_tests"
   },
   {
@@ -364,6 +368,7 @@
       "--revision=170242",
       "webrtc_perf_tests"
     ],
+    "cwd": "[BUILDER_CACHE]/Mac64_Release__large_tests_",
     "name": "webrtc_perf_tests"
   },
   {