nixos/tests: extend shell_interact to accept alternative socat addresses

`shell_interact()` is currently not nice to use.  If you try to cancel
the socat process, it will also break the nixos test. Furthermore
ptpython creates it's own terminal that subprocesses are running in,
which breaks some of the terminal features of socat.
Hence this commit extends `shell_interact` to allow also to connect to
arbitrary servers i.e. tcp servers started by socat.

nixos/lib/test-driver/test_driver/machine.py

@@ -549,18 +549,27 @@ class Machine:
 
         return (rc, output.decode())
 
-    def shell_interact(self) -> None:
-        """Allows you to interact with the guest shell
+    def shell_interact(self, address: Optional[str] = None) -> None:
+        """Allows you to interact with the guest shell for debugging purposes.
 
-        Should only be used during test development, not in the production test."""
+        @address string passed to socat that will be connected to the guest shell.
+        Check the `Running Tests interactivly` chapter of NixOS manual for an example.
+        """
         self.connect()
-        self.log("Terminal is ready (there is no initial prompt):")
+
+        if address is None:
+            address = "READLINE,prompt=$ "
+            self.log("Terminal is ready (there is no initial prompt):")
 
         assert self.shell
-        subprocess.run(
-            ["socat", "READLINE,prompt=$ ", f"FD:{self.shell.fileno()}"],
-            pass_fds=[self.shell.fileno()],
-        )
+        try:
+            subprocess.run(
+                ["socat", address, f"FD:{self.shell.fileno()}"],
+                pass_fds=[self.shell.fileno()],
+            )
+            # allow users to cancel this command without breaking the test
+        except KeyboardInterrupt:
+            pass
 
     def console_interact(self) -> None:
         """Allows you to interact with QEMU's stdin