Improve testing for controllers

`snaps-controllers` tests currently depend on the `snaps-execution-environments` Webpack build. These tests could be improved by running with a mocked execution environment instead, which would remove the need for the `snaps-execution-envrionments` build. It can also speed up tests by a lot.

For this it's important that we have good unit tests in both packages.

Start by inspecting the existing tests in the snaps-controllers package and how they depend on the snaps-execution-environments Webpack build. Determine the mocking boundary and add unit coverage in both packages. Done means the controller tests run with a mocked execution environment, no longer require that build, and retain good coverage.

Written by the indexing model from the issue text.