Test `files upload` command (or deprecate it)

We don't test the `sentry-cli files upload` command. At all (see #2495). We should add tests.

Although, admittedly, I am not really sure why we have this command or if anyone uses it – I'd expect `debug-files upload` and `sourcemaps upload` to be the primary commands used for uploading. Perhaps, we should just deprecate this command, instead of adding tests.