The run-pass and pretty run-pass tests could run concurrently, and if they
do, they need to keep their output segregated.
This change might be overkill. We need the suffix for the `pretty` mode,
but we might not need it otherwise. The `debuginfo-lldb` and
`debuginfo-gdb` modes look like they could also need it, but the current
`tests.mk` file happens not to enable both lldb and gdb at the same time,
for incidental reasons.
38d26d811a