modules/test: provide access to expect function

Allow `test.warnings` and `test.assertions` to be defined as either a
list, or a function coerced to a list.

When defined as a function, it is supplied an `expect` function which
provides some syntactic-sugar for defining simple expectations.

This is an alternative to the current approach of defining that `expect`
function on an ad-hoc basis.

I prefer this to adding `expect` to nixvim's lib because:
1. That would require having access to `lib`
2. IDK where in `lib` such a specialized function should live

tests/test-sources/plugins/by-name/nvim-osc52/default.nix

@@ -1,8 +1,6 @@
 let
-  expect = expect: value: { inherit expect value; };
-
   # This plugin is deprecated
-  warnings = [
+  warnings = expect: [
     (expect "count" 1)
     (expect "any" "this plugin is obsolete and will be removed after 24.11.")
   ];

tests/test-sources/plugins/by-name/rust-tools/default.nix

@@ -1,8 +1,6 @@
 let
-  expect = expect: value: { inherit expect value; };
-
   # This plugin is deprecated
-  warnings = [
+  warnings = expect: [
     (expect "count" 1)
     (expect "any" "The `rust-tools` project has been abandoned.")
   ];