Module source object host invariant test

[guybedford]

This adds a test for source phase imports of the host invariant:

The Module Source Object corresponding to this source Module Record's source phase (), or empty if it is not available for this module kind. When not empty, it must be an object whose initial [[Prototype]] is an object whose initial [[Prototype]] is %AbstractModuleSource%.prototype.

which was recently updated in tc39/proposal-source-phase-imports#77.