lib/test/container-test-driver: Fix extraPythonPackages support

Fix `extraPythonPackages` support in the container test driver.

It triggered the following warning from `nixpkgs`:
```
python3Packages = throw "do not use python3Packages when building Python packages, specify each used package as a separate argument"; # do not remove
```

The following `callPackage` usage triggered the `throw`:

```
hostPackages.python3.pkgs.callPackage
```

The change to a regular `callPackage` i`nvocation fixes this issue.

Added a container test with a popular package to ensure compatibility in the future.

Closes: #5459

checks/flake-module.nix

@@ -97,6 +97,7 @@ in
             nixos-test-container = self.clanLib.test.containerTest ./container nixosTestArgs;
             nixos-test-user-firewall-iptables = self.clanLib.test.containerTest ./user-firewall/iptables.nix nixosTestArgs;
             nixos-test-user-firewall-nftables = self.clanLib.test.containerTest ./user-firewall/nftables.nix nixosTestArgs;
+            nixos-test-extra-python-packages = self.clanLib.test.containerTest ./test-extra-python-packages nixosTestArgs;
 
             service-dummy-test = import ./service-dummy-test nixosTestArgs;
             wireguard = import ./wireguard nixosTestArgs;

checks/test-extra-python-packages/default.nix

@@ -0,0 +1,26 @@
+(
+  { ... }:
+  {
+    name = "test-extra-python-packages";
+
+    extraPythonPackages = ps: [ ps.numpy ];
+
+    nodes.machine =
+      { ... }:
+      {
+        networking.hostName = "machine";
+      };
+
+    testScript = ''
+      import numpy as np
+
+      start_all()
+      machine.wait_for_unit("multi-user.target")
+
+      # Test availability of numpy
+      arr = np.array([1, 2, 3])
+      print(f"Numpy array: {arr}")
+      assert len(arr) == 3
+    '';
+  }
+)