testscript: improve wait_for_ssh() network diagnosis and logging

We will be able to better understand if a network issue is caused by
the VM host or the VM.

Now, in wait_for_ssh() we first wait for ping and then we ssh
--> better diagnosis of guest network state, e.g., ping may
    work but ssh fail

A possible missing IP in VM host error might be reported like this:

```
The VM host is not in the same network as IP 192.168.5.2! Networks:
[IPv4Network('192.168.1.0/24'), IPv4Network('192.168.2.0/24'), IPv4Network('192.168.3.0/24'), IPv4Network('192.168.4.0/24'), IPv4Network('192.168.100.0/24')]
```

On-behalf-of: SAP philipp.schuster@sap.com
Signed-off-by: Philipp Schuster <philipp.schuster@cyberus-technology.de>

Author: phip1611

tests/testscript.py

@@ -1975,11 +1975,10 @@ def wait_until_fail(func):
 
 def wait_for_ssh(machine: Machine, user="root", password="root", ip="192.168.1.2"):
     """
-    Waits for SSH to become available to connect into the Cloud Hypervisor VM
-    hosted on the corresponding machine.
+    Waits for the VM to become accessible via SSH.
 
-    Effectively we use it to wait until the Cloud Hypervisor VM's network is up
-    and available.
+    It first checks whether the VM responds to ping, and then attempts to
+    establish an SSH connection using the provided credentials.
 
     :param machine: VM host
     :param user: user for SSH login
@@ -1988,10 +1987,28 @@ def wait_for_ssh(machine: Machine, user="root", password="root", ip="192.168.1.2
     :return: True if the VM became available in a reasonable amount of time
     """
     retries = 100
+
+    # Sometimes we experienced test runs where the host lost IPs. We therefore
+    # check that early and always for better debuggability.
+    assert_ip_in_local_192_168_net24(machine, ip)
+
+    print(f"Waiting for VM with IP {ip} to become available ...")
+
+    # First wait until the VM is pingable
+    for i in range(retries):
+        print(f"Checking ping to {ip} ({i + 1}/{retries}) ...")
+        if is_pingable(machine, ip, 1, 1):
+            print(f"{ip} is pingable")
+            break
+    else:
+        print(f"{ip} is not reachable via ping after {retries} attempts")
+        return False
+
     for i in range(retries):
-        print(f"Wait for ssh {i}/{retries}")
+        print(f"Wait for SSH ({i + 1}/{retries}) ...")
         status, _ = ssh(machine, "echo hello", user, password, ip)
         if status == 0:
+            print("SSH available")
             return True
         time.sleep(0.1)
     return False