Add getTemplateData caching test

[samuelpath]

In PR #967, I tried to refactor the getTemplateData function in EleventyFiles.js by replacing the if and return by a ternary operator.

My refactor broke the caching but all the tests passed nonetheless.

On closing the PR, @zachleat said:

Would be very interested in a PR that adds a test around this!

So here is the PR that adds a test around this.

What it does is simply call the getTemplateData function twice in a row and compares the return values.

Since the getTemplateData function returns an object, we can compare the object reference by using the is test.

I've been successfully able to break my test by removing the caching mechanism. In that case, the test result says:

Values are deeply equal to each other, but they are not the same

Which is what we would expect without caching.

[zachleat]

Thank you!!