Refactor tests to load flake inputs with flake-compat

This makes `nix fmt` just works and we no longer have to override flake
inputs.

tests/run-tests.nix

@@ -0,0 +1,19 @@
+{
+  lib,
+  writeShellScriptBin,
+  bash,
+  python3,
+  nix-eval-jobs,
+  self,
+}:
+
+writeShellScriptBin "run-tests" ''
+  #!${bash}/bin/bash
+  export PATH=${
+    lib.makeBinPath [
+      nix-eval-jobs
+      nix-eval-jobs.nix
+    ]
+  }
+  exec ${python3.interpreter} ${self}/tests/run.py
+''