Add tests for position of module declarations

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.