pythonPackages.tornado: fix tests, closes #14634

The test runner would import all modules to check whether they might
have tests.

On a Linux system it would load the Tornado module that depends on
kqueue, which is BSD only, and fail.

pkgs/top-level/python-packages.nix

@@ -24738,10 +24738,12 @@ in modules // {
 
     propagatedBuildInputs = with self; [ backports_ssl_match_hostname_3_4_0_2 certifi ];
 
-    # Tests fail:
-    # ValueError: _type_ 'v' not supported
-    # See https://github.com/NixOS/nixpkgs/issues/14634
-    doCheck = false;
+    # We specify the name of the test files to prevent
+    # https://github.com/NixOS/nixpkgs/issues/14634
+    checkPhase = ''
+      ${python.interpreter} -m unittest discover *_test.py
+    '';
+
     src = pkgs.fetchurl {
       url = "mirror://pypi/t/tornado/${name}.tar.gz";
       sha256 = "a16fcdc4f76b184cb82f4f9eaeeacef6113b524b26a2cb331222e4a7fa6f2969";