Refactor AssignmentExpression tests for parsers

A number of tests for the parsing of the AssignmentExpression production
were expressed using `eval`. This made the tests more complex than
necessary, and also prevented the tests from providing value to
ECMAScript parsers.

Remove the use of `eval` in the relevant tests and instead express the
expectations with literal source text. Remove superfluous "onlyStrict"
restriction from tests by declaring the probe binding prior to
assignment.