live testing for Firebase parity

[mhuebert]

I suggest we set up async tests that run against a live Firebase database. We currently have a number of known inconsistencies, and Firebase rule evaluation itself can change without notice.

[dinoboff]

#91 has some limitation:

• It uses the REST API.
• it uses the legacy secret key authentication.
• It's very slow to run. It only record evaluation of some rules from a live Firebase DB in test/spec/lib/parser/fixtures.json.
• it doesn't test read or write operation. It only test how a rule is parsed and evaluated by Firebase; it doesn't test which rules should be read during a read/write operation.
• the rule is tested in a read operation context; it doesn't have access to newData.
• the path of rule getting parsed is not predictable when using wildchildren; it should only use root to test snapshot behaviours.

@mhuebert is it ok?

[dinoboff]

There's now a daily cron job checking behaviour changes targaryen assumptions about the parser.

It running on travis and slow the test by 30s only. I tried on semaphoreci which has better cron support by requests to firebase is slow; the tests were taken +4min to run.

[mhuebert]

Very happy to see this in here! 👍