* 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.
Add an option to set custom `$ZPLUG_HOME`. Changing it with
`home.sessionVariables` doesnt work, since it has to be exported
before Zplug is initialised
This option provides a more convenient way to overlay dummy packages.
It also adds a function `config.lib.test.mkStubPackage` that can,
e.g., be used for `package` options.
Running `zplug install` will always product output, even if there is
nothing to do.
Gating it behind a `zplug check` eliminates that output when there is
nothing to do, and is recommended in the zplug README.
