This Answer Record demonstrates how to use the testbench generator tool in the Design Utilities in the Xilinx TcL store, which provides a clock and reset stimulus.
To use this utility, go to Tools -> Xilinx Tcl Store, and select Refresh.
Once, the refresh is done. Install Design Utilities 1.17 (or later):
Open your Block Design (BD), and run the Tcl command below to generate the testbench: