Use `mkTestDerivationFromNixvimModule` instead of `mkTestDerivation`,
allowing "proper" modules to be used instead of plain attr configs.
This is useful for more complex tests that wish to use `config` or
`options` arguments, e.g:
```nix
{config, options, ...}: {
/* some cool test */
}
```
To allow `tests.dontRun` to be defined on such a test, the module is
allowed to be nested as `module`, e.g:
```nix
{
tests.dontRun = true;
module = {config, options, ...}: {
/* a disabled test */
};
}
```
Also ended up doing some general cleanup, removing an unused function,
etc.
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
