convert: modernize tests

convert: modernize tests

Use `test_config` to set the config, check that files are empty with
`test_must_be_empty`, compare files with `test_cmp`, and remove spaces
after ">" and "<".

Please note that the "rot13" filter configured in "setup" keeps using
`git config` instead of `test_config` because subsequent tests might
depend on it.

Reviewed-by: Stefan Beller <redacted>
Signed-off-by: Lars Schneider <redacted>
Signed-off-by: Junio C Hamano <redacted>