It seems the way to go at the moment is hardcoding include statements as special comments in the suite file (called doc-comments references), e.g.:
// include external files like so:
/// <reference path="/path/to/external-file.js" />
// than write your testing suite as usual:
describe('a silly suite', function() {
it('should test something is happening', function() {
expect(something).toBe('happening');
});
});
See this thread on the ReSharper community, as the source of this recommendation.