flake: Expose tests to allow running purely (#3412)

* flake: Expose tests to allow running purely

The existing way to run tests with `nix-shell` relies on impure usage of
`<nixpkgs>`. This can lead to failures when the local nixpkgs is
incompatible with the locked one. I.e., where CI is passing but a
contributor may experience a failure.

So, expose tests as `devShells.tests` to use the locked nixpkgs and
allow easy invocation via `nix develop`.

* tests: Remove impure path

With Nix 2.10+ and pure evaluation mode e.g.

```
nix run nixpkgs/nixos-unstable#nixVersions.nix_2_10 -- develop -i .#tests.zplug-modules
```

this test would fail with:

> error: the path '~/.customZplugHome' can not be resolved in pure mode

Since the test only cares that it is a path, rather than anything about
its contents, use a dummy empty directory.

docs/contributing.adoc

@@ -256,3 +256,8 @@ and run an individual test, for example `alacritty-empty-settings`, through
 
 [source,console]
 $ nix-shell --pure tests -A run.alacritty-empty-settings
+
+However, those invocations will impurely source the system’s nixpkgs, and may cause failures. To run against the nixpkgs from the flake.lock, use instead e.g.
+
+[source,console]
+$ nix develop --ignore-environment .#tests.all

flake.nix

@@ -88,7 +88,9 @@
       let
         pkgs = nixpkgs.legacyPackages.${system};
         docs = import ./docs { inherit pkgs; };
+        tests = import ./tests { inherit pkgs; };
       in {
+        devShells.tests = tests.run;
         packages = rec {
           home-manager = pkgs.callPackage ./home-manager { };
           docs-html = docs.manual.html;

tests/modules/programs/zplug/modules.nix

@@ -8,7 +8,7 @@ with lib;
       enable = true;
       zplug = {
         enable = true;
-        zplugHome = ~/.customZplugHome;
+        zplugHome = pkgs.emptyDirectory;
         plugins = [
           {
             name = "plugins/git";