login test: Create and use direct reads of the TTY contents.

nixos/lib/test-driver/Machine.pm

@@ -504,6 +504,31 @@ sub screenshot {
     }, { image => $name } );
 }
 
+# Get the text of TTY<n>
+sub getTTYText {
+    my ($self, $tty) = @_;
+
+    my ($status, $out) = $self->execute("fold -w 80 /dev/vcs${tty}");
+    return $out;
+}
+
+# Wait until TTY<n>'s text matches a particular regular expression
+sub waitUntilTTYMatches {
+    my ($self, $tty, $regexp) = @_;
+
+    $self->nest("waiting for $regexp to appear on tty $tty", sub {
+        retry sub {
+            return 1 if $self->getTTYText($tty) =~ /$regexp/;
+        }
+    });
+}
+
+# Debugging: Dump the contents of the TTY<n>
+sub dumpTTYContents {
+    my ($self, $tty) = @_;
+
+    $self->execute("fold -w 80 /dev/vcs${tty} | systemd-cat");
+}
 
 # Take a screenshot and return the result as text using optical character
 # recognition.