nixos/tests: Set DefaultTimeoutStartSec very high (#44916)

DefaultTimeoutStartSec is normally set to 90 seconds and works fine. But
when running NixOS tests on a very slow machine (like a VM without
nested virtualisation support) this default is to low and causes
systemd units to fail spuriously. One symptom of this issue are tests
at times failing with "timed out waiting for the VM to connect".

Since the VM connect timeout is 300 seconds I also set
DefaultTimeoutStartSec to this which is ridiculously high.

nixos/modules/testing/test-instrumentation.nix

@@ -102,8 +102,12 @@ with import ../../lib/qemu-flags.nix { inherit pkgs; };
         MaxLevelConsole=debug
       '';
 
-    # Don't clobber the console with duplicate systemd messages.
-    systemd.extraConfig = "ShowStatus=no";
+    systemd.extraConfig = ''
+      # Don't clobber the console with duplicate systemd messages.
+      ShowStatus=no
+      # Allow very slow start
+      DefaultTimeoutStartSec=300
+    '';
 
     boot.consoleLogLevel = 7;