tests: move package to its own file

Extract the `tests` package from `flake.nix`  into its own file
`tests/package.nix`.

This a) de-couples it from the flake (to a degree) and b) allows more
neatly using the callPackage pattern.

flake.nix

@@ -80,9 +80,7 @@
               ./modules/misc/news/create-news-entry.sh
             '';
 
-            tests = pkgs.writeShellScriptBin "tests" ''
-              exec ${pkgs.python3}/bin/python3 ${self}/tests/tests.py "$@"
-            '';
+            tests = pkgs.callPackage ./tests/package.nix { flake = self; };
 
             docs-html = docs.manual.html;
             docs-htmlOpenTool = docs.manual.htmlOpenTool;

tests/package.nix

@@ -0,0 +1,8 @@
+{
+  flake,
+  python3,
+  writeShellScriptBin,
+}:
+writeShellScriptBin "tests" ''
+  exec ${python3}/bin/python3 ${flake}/tests/tests.py "$@"
+''