Add tests for Lexical Environment management

Add tests that assert the management of the running execution context's
LexicalEnvironment and VariableEnvironment components, as created by the
following abstract operations:

- NewDeclarativeEnvironment
- NewObjectEnvironment
- NewFunctionEnvironment

Many tests require the use of non-strict direct eval, meaning they may
not be run in strict mode. This does not effect coverage because the
semantics in these cases are not observable from strict mode code.

Some situations require the creation of a binding, but this binding has
no relevance to the test itself. In these cases, use names consisting
solely of the underscore character (`_`).

Avoid the use of Block statements wherever possible, as these trigger
the creation of additional environments which may interfere with the
behavior under test.