tests: convert bare tests to bootstrap tests

The only script that is not executed in a specific environment is bootstrap.lua and that requires os.getenv to work and our tests are designed to run in sanbox mode only where this is not available. Thus we do not test this script and anything else is run in environment declared in router's entry script or in bootstrap.lua.

The testing now checks scripts in the environment that is same as it would be when bootstrapping. This should discover errors such as when os_release is unexpectedly empty.