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

lib/utils.nix

@@ -1,9 +1,14 @@
-{lib}:
+{
+  lib,
+  _nixvimTests,
+}:
 with lib; {
   listToUnkeyedAttrs = list:
     builtins.listToAttrs
     (lib.lists.imap0 (idx: lib.nameValuePair "__unkeyed-${toString idx}") list);
 
+  enableExceptInTests = !_nixvimTests;
+
   emptyTable = {"__empty" = null;};
 
   /*