Implement suggested modification to naming scheme.

Files whose name ends in `_.js` are not themselves valid Test262 tests
and should not be interpreted as such by test runners.

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The tests for the GetBindingValue method of the module Environment
Record are very minimal. This is because GetBindingValue is necessary to
assert any aspect of binding creation/initialization/mutation. In this
way, GetBindingValue is being implicitly tested by every test that
references a binding value.

The project's expected frontmatter tag name changed while these files
were under review.

Assert that ImportDeclaration and ExportDeclaration match only the
ModuleItem symbol.

According to the definition of HostResolveImportedModule, it is
acceptable for an implementation to throw a SyntaxError in the event
that a requested module can neither be found nor created:

> If a Module Record corresponding to the pair referencingModule,
> specifier does not exist or cannot be created, an exception must be
> thrown.

In order to reliably detect a SyntaxError in response to the correct
interpretation of the grammar (and not a SyntaxError from an *incorrect*
interpretation of the grammar followed by a failure to resolve the
requested module), the ModuleSpecifier of ExportDeclarations should
describe a valid resource.