Add a test that uses SyncPackageResolver.loadConfig which also works

[zoechi]

with relative paths in .packages

[nex3]

I'm not sure I understand why we're testing this here. Isn't it the responsible for the Dart infrastructure—pkg_config and package_resolver—to ensure that the resolver behaves correctly?

[zoechi]

I'm not sure either, but I wasted quite some time to debug dart-sass to get transform_sass working until it turned out the issue is caused by how the .packages config is read.
I built the test for debugging and thought it might be valuable to others to see how the resolver needs to be initialized for this scenario to work.
If you rather not have this in tests, feel free to ignore the PR.