[Test] Use real instance of ReservedRolesStore to avoid NPE (elastic#97185)

Since ReservedRolesStore must be properly initialized before calling real methods,
it is not possible to mock it and then still call real methods. Given that there is no real
need for CompositeRolesStoreTests to use mocked ReservedRolesStore, this PR
changes the test setup to use real instance and thus avoid NPE when accessing
RESERVED_ROLES property.

Fixes elastic#97172

Author: slobodanadamovic

x-pack/plugin/security/src/test/java/org/elasticsearch/xpack/security/authz/store/CompositeRolesStoreTests.java

@@ -2920,8 +2920,7 @@ private RoleProviders buildRolesProvider(
             }).when(nativeRolesStore).getRoleDescriptors(isASet(), anyActionListener());
         }
         if (reservedRolesStore == null) {
-            reservedRolesStore = mock(ReservedRolesStore.class);
-            doCallRealMethod().when(reservedRolesStore).accept(anySet(), anyActionListener());
+            reservedRolesStore = new ReservedRolesStore();
         }
         if (licenseState == null) {
             licenseState = new XPackLicenseState(() -> 0);