* 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.
cc58d31953