tests: Introduce a way to disable options when running in tests (#1095)

In our basic template we used to provide a check based on
`mkTestDerivationFromNvim`. The issue with this check (that is handled
correctly internally) is that some plugins _can't_ be used in the test
environment, for example image.nvim like in #1085.

This commit introduces a new function to generate such checks,
`mkTestDerivationFromNixvimModule`, that wraps a nixvim configuration
instead of a built nvim instance.

Then a configuration can rely on the newly added
`helpers.enableExceptInTests` attribute to disable parts of the
configuration depending if it is evaluated in tests or in a real final
configuration.

Resolves #1085

docs/helpers.md

@@ -37,3 +37,6 @@ A certain number of helpers are defined that can be useful:
   ```lua
     {"a", "b", [foo] = "bar"}
   ```
+
+- `helpers.enableExceptInTests`: Evaluates to `true`, except in `mkTestDerivationFromNixvimModule`
+  where it evaluates to `false`. This allows to skip instantiating plugins that can't be run in tests.