nixos tests: waitUntilTTYMatches: Log TTY contents on last try

If the test has not passed yet, on the last attempt it now outputs:

    machine: Last chance to match /logine: / on TTY2, which currently contains:
    machine: running command: fold -w$(stty -F /dev/tty2 size | awk '{print $2}') /dev/vcs2
    machine: exit status 0
    machine:

    <<< Welcome to NixOS 17.09.git.a804ef4 (x86_64) - tty2 >>>

    machine login:

to help debug the problem. Notice the "logine" typo in my check.

nixos/lib/test-driver/Machine.pm

@@ -518,6 +518,12 @@ sub waitUntilTTYMatches {
 
     $self->nest("waiting for $regexp to appear on tty $tty", sub {
         retry sub {
+            my ($retries_remaining) = @_;
+            if ($retries_remaining == 0) {
+                $self->log("Last chance to match /$regexp/ on TTY$tty, which currently contains:");
+                $self->log($self->getTTYText($tty));
+            }
+
             return 1 if $self->getTTYText($tty) =~ /$regexp/;
         }
     });