Allow npm and yarn support for testing.

[Aalekh Patel]

Consider the scripts in the root package.json:

{
"scripts": {
    "test:auth": "cd modules/auth && yarn test",
    "test:database": "cd modules/database && yarn test",
    "test:firestore": "cd modules/firestore && yarn test",
    "test": "yarn test:auth && yarn test:database && yarn test:firestore"
  },
}

Problem

If someone does not have yarn installed and works exclusively with npm then all the scripts FAIL.

Suggested fix

Probably a better idea to either default to npm or else have the keys say : test:auth:npm and test:auth:yarn, and handle the scripts accordingly.