Improve address translation tests (#261)

* Improve address translation tests.

Check that the mode we're testing is supported by hardware before
running the test.
Test with high address bits set, which catches a bug in OpenOCD.

* Turn off PMP for address translation test.

Otherwise it doesn't pass on HiFive Unleashed.

* Run TranslateTest on random hart.

Once https://github.com/riscv/riscv-openocd/pull/459 merges that will work.