[fuchsia] Test SSH connectivity before attempting reproduction. (#1004)
[fuchsia] Test SSH connectivity before attempting reproduction. (#1004)

We've been seeing SSH connection refused problems when running
reproducers (but not when running fuzzers); in an effort to root-cause
the problem, let's actually test the ssh connection before connecting.

If we notice CF dies at test_qemu_ssh() consistently, it's a strong
signal that perhaps the QEMU VM is dying prematurely.

If we notice CF dies *after* test_qemu_ssh() consistently, it's a signal
that perhaps something about our specific ssh call while running
reproducers is incorrect.
google/clusterfuzz,google/clusterfuzz,google/clusterfuzz,google/clusterfuzz,google/clusterfuzz,google/clusterfuzz,google/clusterfuzz,google/clusterfuzz