Testbench cleanup

Created by: jordancarlin

• Use VCS instead of SIM_VCS for vas related ifdefs in testbench as VCS is enabled by default in the simulator and does not need to be passed as an argument
• Base $stop vs $finish off of the QUESTA macro instead of VCS and VERILATOR since Questa seems to be the odd one out and this only requires one ifdef instead of two
• Correct usage of S_SUPPORTED in testbench_fp and replace with F_SUPPORTED