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.

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";