So you have two options here. Like you said yourself writing isolated unit tests would probably test enough already for the package. The one that integrates with your package can then decide if they want to write any tests against that.
The other option is stripping your package more to a smaller versions where it doesn't depend on all these things. Maybe your package should be split into multiple smaller package so they are testable and so on.
Looking in the testbench documentation you can also perform the migrations you want to perform: https://github.com/orchestral/testbench#using-migrations