Add tests for script interactions (#689)

ECMA262 allows for an arbitrary number of "ScriptJob"s to run in a given realm.
Although there is no standard mechanism for enqueuing these jobs, many
implementations offer this functionality through custom APIs. In those hosts,
the semantics describing script interactions are directly observable.
In order to guarantee conformance to the specification in advance of a
standardized API, Test262 now requires that hosts provide a $.evalScript
function whose behavior is defined in the project's "INTERPRETING.md" file.
Use this host-provided API to ensure that implementations correctly observe the
specification text that dictates script interactions.
(In writing these tests, I noticed some gaps in coverage that are observable
from a single script execution. This patch includes a dedicated commit for
these tests that do not require $.evalScript.)

* Improve coverage of GlobalDeclarationInstantiation

* Add tests for script interactions

Use the host-provied `$.evalScript` method to assert conformance to the
specification text that defines script interactions.

* fixup! Improve coverage of GlobalDeclarationInstantiation