Fix test:node:persistence command. (#1688)

Since our code is TypeScript we use `--require ts-node/register` for mocha to
be able to run our code. We have this require in our global
`../../config/mocha.node.opts` file, but it apparently isn't applied before the
`--require test/util/node_persistence.ts` in our test:node:persistence
command-line. So I've had to add an explicit `--require ts-node/register` to
our command-line.

Author: mikelehen

packages/firestore/package.json

@@ -15,7 +15,7 @@
     "test:browser": "karma start --single-run",
     "test:browser:debug": "karma start --browsers=Chrome --auto-watch",
     "test:node": "TS_NODE_CACHE=NO TS_NODE_COMPILER_OPTIONS='{\"module\":\"commonjs\"}' nyc --reporter lcovonly -- mocha 'test/{,!(browser)/**/}*.test.ts' --file index.node.ts --opts ../../config/mocha.node.opts",
-    "test:node:persistence": "USE_MOCK_PERSISTENCE=YES TS_NODE_CACHE=NO TS_NODE_COMPILER_OPTIONS='{\"module\":\"commonjs\"}' nyc --reporter lcovonly -- mocha 'test/{,!(browser)/**/}*.test.ts' --file index.node.ts --require test/util/node_persistence.ts --opts ../../config/mocha.node.opts",
+    "test:node:persistence": "USE_MOCK_PERSISTENCE=YES TS_NODE_CACHE=NO TS_NODE_COMPILER_OPTIONS='{\"module\":\"commonjs\"}' nyc --reporter lcovonly -- mocha 'test/{,!(browser)/**/}*.test.ts' --require ts-node/register --file index.node.ts --require test/util/node_persistence.ts --opts ../../config/mocha.node.opts",
     "test:emulator": "ts-node ../../scripts/emulator-testing/firestore-test-runner.ts",
     "prepare": "npm run build"
   },