Add E2E test to verify single language enforcement

[pragnagopa]

E2E test that verifies enforcing single language was removed as part of refactoring:
https://github.com/Azure/azure-functions-host/pull/3425/files#diff-ccc382a242c71486b04a04a9eb3c1790

This is a true E2E test and will help us catch regressions

[pragnagopa]

cc: @mathewc