Fixes #938. Remove "esnext: true" flags from tests that should be "moz: true"

In addition to removing the erroneous flags:

- Consolidates all the esnext & moz tests into a single `isMozOrESNext()` function that returns true if either flag is set.
- Allows an esnext flag to override a moz flag when both are present (the former will be a standard)
- Disallows "feature" overlap where such feature does not have the the same syntactic form across moz and esnext (eg. array comprehension syntax)

Signed-off-by: Rick Waldron <waldron.rick@gmail.com>