Parser conformance test suite

[JimLarson]

Create a new conformance test suite for the parser, subsuming some of the functionality in the google/cel-go parser tests. The "simple" end-to-end tests always fail on parse errors, by design.

In such a suite, test that the non-constant reserved identifiers cause parse failures: as, break, const, continue, else, for, function, if, import, in, let, loop, package, namespace, return, var, void, while.