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

···
 , enableInfiniBandRdma ? false
 , enableMonitoring ? false
 , enableSnmp ? false
+, nixosTests
 }:
 
 with lib;