corosync: fix tests eval

Without the change attempt to run tests fails as:

    $ nix build --no-link -f. corosync.tests
    error: undefined variable 'nixosTests'
           68|   passthru.tests = {
           69|     inherit (nixosTests) pacemaker;
             |              ^
           70|   };

pkgs/servers/corosync/default.nix

@@ -4,6 +4,7 @@
 , enableInfiniBandRdma ? false
 , enableMonitoring ? false
 , enableSnmp ? false
+, nixosTests
 }:
 
 with lib;