Vex RISC-V JTAG bridge port choice

[rslawson]

Currently with the most recent PR merging, the unit tests for clash-vexriscv-sim have to be run with -j1 since they both use port 7894 for the JTAG bridge. When I attempted to use other ports for this, the tests simply failed and without any debug output that might suggest a problem. So, we need to figure out how to get the JTAG bridge to run on multiple ports so that the tests can be run in parallel instead of serially.