test/test_qemu_running_processes: make timeout actually work (!1886)

Just counting the number of tries does not prevent the QEMU tests
from running long periods of time. In some situations, the ssh command
will keep trying to reach the destination almost indefinitely.

To make the 5 minutes timeout work correctly we need to:
  - Add an appropriate ConnectTimeout to the ssh command
  - Don't sleep for another second if ssh already took several seconds
  - Check the elapsed time instead of setting a fixed amount of tries

test/test_qemu_running_processes.py

@@ -109,6 +109,7 @@ def ssh_run(args, command):
     :returns: the result from the SSH server
     """
     ret = pmb.chroot.user(args, ["SSH_ASKPASS=/tmp/y.sh", "DISPLAY=", "ssh",
+                                 "-o", "ConnectTimeout=10",
                                  "-o", "UserKnownHostsFile=/dev/null",
                                  "-o", "StrictHostKeyChecking=no",
                                  "-p", "2222", "testuser@localhost", "--",
@@ -116,24 +117,30 @@ def ssh_run(args, command):
     return ret
 
 
-def is_running(args, programs, tries=300, sleep_before_retry=1):
+def is_running(args, programs, timeout=300, sleep_before_retry=1):
     """
     Simple check that looks for program names in the output of "ps ax".
     This is error-prone, only use it with programs that have a unique name.
-    With defaults retries and sleep_before_retry values, it will try each
-    second for 5 minutes.
+    With defaults timeout and sleep_before_retry values, it will try keep trying
+    for 5 minutes, but not more than once per second.
 
     :param programs: list of programs to check for, e.g. ["xfce4-desktop"]
-    :param tries: amount of tries with the wrong result before giving up
+    :param timeout: approximate time in seconds until timeout
     :param sleep_before_retry: time in seconds to sleep before trying again
     """
-    print("Looking for programs to appear in the VM (tries: " + str(tries) +
+    print("Looking for programs to appear in the VM (timeout: " + str(timeout) +
           "): " + ", ".join(programs))
     ssh_works = False
-    for i in range(0, tries):
-        # Sleep
-        if i > 0:
-            time.sleep(sleep_before_retry)
+
+    end = time.monotonic() + timeout
+    last_try = 0
+
+    while last_try < end:
+        # Sleep only when last try exited immediately
+        sleep = last_try - time.monotonic() + sleep_before_retry
+        if sleep > 0:
+            time.sleep(sleep)
+        last_try = time.monotonic()
 
         # Get running programs via SSH
         all = ssh_run(args, "ps ax")