Tightened the arrow function tests, version 2

This is an alternative approach to #168. The discussion for that PR emphasised that the tightened test, while more thorough, was worrisomely monolithic and tended to obscure partial functionality. So, this PR simply does the following:
* Adds just the parameter checks to the basic "arrow functions" test
* Adds a separate test, "lexical "this" in arrow functions", which checks that arrows have a bound `this` that can't be re-bound.

Unlike #168, this does not include the following "pedantic" checks, which, while important for conformance, are less important for the syntax's general day-to-day usage:
* Lexical `arguments`
* No line terminator between parameters and `=>`
* Absence of `prototype` property (throws on `new`), and empty `name` property