Rename "make testbench.vcd" to "make testbench_vcd" so VCD file is not removed on Ctrl-C
Rename "make testbench.vcd" to "make testbench_vcd" so VCD file is not removed on Ctrl-C

cliffordwolf/picorv32,cliffordwolf/picorv32,cliffordwolf/picorv32,cliffordwolf/picorv32