betamax: Disable tests

As said in #15037, the tests fail due to a network error as the tests
expect to have network access.

This simply disables the tests, which is not optimal, of course. I do
not understand the whole python infrastructure, so I'm not able to
disable only the failing-because-of-network-access tests.

pkgs/top-level/python-packages.nix

@@ -1717,6 +1717,8 @@ in modules // {
 
     propagatedBuildInputs = [ self.requests2 ];
 
+    doCheck = false;
+
     meta = with stdenv.lib; {
       homepage = https://betamax.readthedocs.org/en/latest/;
       description = "A VCR imitation for requests";