debug: print failures when debug tests fail, so we can see why it is failing on Travis

Cleanups, and print out log names ASAP.

Factor out gdbserver common invocation into GDBSERVER (fixing
--print-failtures).
Add --print-log-names to that command so the logfiles can be inspected
while the simulation is still running.
`RISCV=... cmd` is more idiomatic than `export RISCV=... && cmd`