asset_tweak::tweak_expect_or_create works by writing default data to
file and then read it as asset.
the problem is that it was writing T, and read AssetTweakWrapper<T>
which are different types.
Tests didn't handle case when you will load data back so bug was hidden.
- Use `serial_test` because tests can't run in parallel as both of them
are accessing fs.
- Take only filename, use hardcoded `assets/tweak` to keep it simple and
support .gitignore
