nixos/lemurs: Fix test

> machine.wait_until_succeeds("pgrep -f 'lemurs.*tty1'")

This was guaranteed to succeed whether the desired process existed or
not, because it was matching the 'bach -c ...' command the test driver
uses to run the command. Let's wait for evidence on screen instead.

Changed files
+3 -1
nixos
tests
lemurs
+3 -1
nixos/tests/lemurs/lemurs.nix
···
];
};
+
enableOCR = true;
+
nodes.machine = _: {
imports = [ ../common/user-account.nix ];
services.displayManager.lemurs.enable = true;
···
machine.start()
machine.wait_for_unit("multi-user.target")
-
machine.wait_until_succeeds("pgrep -f 'lemurs.*tty1'")
+
machine.wait_for_text("Login")
machine.screenshot("postboot")
with subtest("Log in as alice on a virtual console"):