Python: introduce tests for interpreters

This adds tests to the passthru of all Python interpreters.

pkgs/development/interpreters/python/tests.nix

@@ -0,0 +1,63 @@
+{ python
+, runCommand
+, substituteAll
+, lib
+}:
+
+let
+  envs = let
+    inherit python;
+    pythonEnv = python.withPackages(ps: with ps; [ ]);
+  in {
+    # Plain Python interpreter
+    plain = rec {
+      env = python;
+      interpreter = env.interpreter;
+      is_venv = "False";
+      is_nixenv = "False";
+    };
+  } // lib.optionalAttrs (python.implementation != "graal") {
+    # Python Nix environment (python.buildEnv)
+    nixenv = rec {
+      env = pythonEnv;
+      interpreter = env.interpreter;
+      is_venv = "False";
+      is_nixenv = "True";
+    };
+  } // lib.optionalAttrs (python.isPy3k && (!python.isPyPy)) rec {
+    # Venv built using plain Python
+    # Python 2 does not support venv
+    # TODO: PyPy executable name is incorrect, it should be pypy-c or pypy-3c instead of pypy and pypy3.
+    plain-venv = rec {
+      env = runCommand "${python.name}-venv" {} ''
+        ${python.interpreter} -m venv $out
+      '';
+      interpreter = "${env}/bin/${python.executable}";
+      is_venv = "True";
+      is_nixenv = "False";
+    };
+    # Venv built using Python Nix environment (python.buildEnv)
+    # TODO: Cannot create venv from a  nix env
+    # Error: Command '['/nix/store/ddc8nqx73pda86ibvhzdmvdsqmwnbjf7-python3-3.7.6-venv/bin/python3.7', '-Im', 'ensurepip', '--upgrade', '--default-pip']' returned non-zero exit status 1.
+    # nixenv-venv = rec {
+    #   env = runCommand "${python.name}-venv" {} ''
+    #     ${pythonEnv.interpreter} -m venv $out
+    #   '';
+    #   interpreter = "${env}/bin/${pythonEnv.executable}";
+    #   is_venv = "True";
+    #   is_nixenv = "True";
+    # };
+  };
+
+  testfun = name: attrs: runCommand "${python.name}-tests-${name}" ({
+    inherit (python) pythonVersion;
+  } // attrs) ''
+    cp -r ${./tests} tests
+    chmod -R +w tests
+    substituteAllInPlace tests/test_python.py
+    ${attrs.interpreter} -m unittest discover --verbose tests #/test_python.py
+    mkdir $out
+    touch $out/success
+  '';
+
+in lib.mapAttrs testfun envs