Fix MLIR build with MLIR_INCLUDE_TESTS=OFF

error message

/usr/bin/ld: CMakeFiles/mlir-opt.dir/mlir-opt.cpp.o: in function `main':
mlir-opt.cpp:(.text.startup.main+0xb9): undefined reference to `mlir::registerTestDialect(mlir::DialectRegistry&)'

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D86592

mlir/tools/mlir-opt/mlir-opt.cpp

@@ -133,7 +133,9 @@ int main(int argc, char **argv) {
 #endif
   DialectRegistry registry;
   registerAllDialects(registry);
+#ifdef MLIR_INCLUDE_TESTS
   registerTestDialect(registry);
+#endif
   return failed(MlirOptMain(argc, argv, "MLIR modular optimizer driver\n",
                             registry,
                             /*preloadDialectsInContext=*/false));