nixos/test-driver: Add new waitForText function.

As promised in the previous commit, this can be used similarly to
$machine->waitForWindow, where you supply a regular expression and it's
retrying OCR until the regexp matches.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

nixos/doc/manual/development/writing-nixos-tests.xml

@@ -244,6 +244,13 @@ startAll;
     connections.</para></listitem>
   </varlistentry>
 
+  <varlistentry>
+    <term><methodname>waitForText</methodname></term>
+    <listitem><para>Wait until the supplied regular expressions matches
+    the textual contents of the screen by using optical character recognition
+    (see <methodname>getScreenText</methodname>).</para></listitem>
+  </varlistentry>
+
   <varlistentry>
     <term><methodname>waitForWindow</methodname></term>
     <listitem><para>Wait until an X11 window has appeared whose name

nixos/lib/test-driver/Machine.pm

@@ -518,6 +518,17 @@ sub getScreenText {
 }
 
 
+# Wait until a specific regexp matches the textual contents of the screen.
+sub waitForText {
+    my ($self, $regexp) = @_;
+    $self->nest("waiting for $regexp to appear on the screen", sub {
+        retry sub {
+            return 1 if $self->getScreenText =~ /$regexp/;
+        }
+    });
+}
+
+
 # Wait until it is possible to connect to the X server.  Note that
 # testing the existence of /tmp/.X11-unix/X0 is insufficient.
 sub waitForX {