Fix tests related to scaffolding generator with --assets=false switch

rails@f1805a697831b redefined `test_scaffold_generator_no_assets` method twice.

Only the last definition was getting called from the test suite.

Due to this, a bug in the test with --assets=false was not discovered.

This PR gives different names to all 3 instances of `test_scaffold_generator_no_assets` and fixes the test with --assets=false switch.