Remove `_verbose` and clean `makeModuleSetUpAndTestCaseClass`.

The `_verbose` attribute of `SlapOSInstanceTestCase` was unused and it
is now removed.
As the `makeModuleSetUpAndTestCaseClass` needed to be edited to remove
`_verbose` from the class type instantiation, that function has been
formatted, documented and typed.

See merge request nexedi/slapos.core!664