revert to using root mocha

Author: Brian Chen

.vscode/launch.json

@@ -26,7 +26,7 @@
       "type": "node",
       "request": "launch",
       "name": "Firestore Unit Tests (Node)",
-      "program": "${workspaceRoot}/packages/firestore/node_modules/.bin/_mocha",
+      "program": "${workspaceRoot}/node_modules/.bin/_mocha",
       "cwd": "${workspaceRoot}/packages/firestore",
       "args": [
         "--require", "ts-node/register/type-check",
@@ -46,7 +46,7 @@
       "type": "node",
       "request": "launch",
       "name": "Firestore Unit Tests (Node / Persistence)",
-      "program": "${workspaceRoot}/packages/firestore/node_modules/.bin/_mocha",
+      "program": "${workspaceRoot}/node_modules/.bin/_mocha",
       "cwd": "${workspaceRoot}/packages/firestore",
       "args": [
         "--require", "ts-node/register/type-check",