Switch to bats over pytest

This removes python entirely from the stack.
Yet to run the tests on macOS, but they're buildable by way of nix
wrapping. Worst case scenario, you cannot run them from the devshell
but `nix run .#test-runner-<latest||stable>` should work okay.

shell.nix

@@ -1,14 +1,30 @@
 {
   pkgs ? import <nixpkgs> { },
-  packages ? [ ],
+  treefmt ? null,
+  nix-direnv ? (pkgs.callPackage ./default.nix { }),
+  test_pkgs ? (pkgs.lib.callPackagesWith pkgs ./tests { inherit nix-direnv; }),
 }:
-
-with pkgs;
-mkShell {
-  packages = packages ++ [
-    python3.pkgs.pytest
-    python3.pkgs.mypy
-    ruff
-    direnv
-  ];
+let
+  inherit (pkgs) lib;
+in
+pkgs.mkShell {
+  DIRENV_STDLIB = "${test_pkgs.direnv-stdlib}";
+  DIRENVRC = "${nix-direnv}/share/nix-direnv/direnvrc";
+  BATS_LIB_PATH = lib.strings.makeSearchPath "" (
+    with test_pkgs;
+    [
+      bats-support
+      bats-assert
+    ]
+  );
+  packages =
+    (builtins.attrValues {
+      inherit (pkgs)
+        bats
+        direnv
+        shellcheck
+        ;
+    })
+    ++ (builtins.attrValues (lib.attrsets.filterAttrs (name: _val: name != "direnv-stdlib") test_pkgs))
+    ++ lib.optionals (treefmt != null) [ treefmt ];
 }