Deliberate overriding of an existing file to tests

Deliberate overriding via `save_as` metadata should be allowed, even after the
“no URL override” feature. This commit is to add tests for deliberate
overriding. As a result, the relevant tests *should fail* after this commit.

Overriding feature implemented in d0e9c52410e70e14008d2ef827ba7ac306243e72
No URL override feature implemented in ff7410ce2ada85b486a67ae11874d60d135ff939