yocto/oeqa: Increase PtestRunnerDom0Test test timeout

The OEQA SSH runner kills processes that do not output anything after a
timeout period (default 300s). If this timeout occurs, the exit code
will be either 255 (if the SSH process exits cleanly) or 0 (if the SSH
process does not terminate within 5s and has to be killed).

The BATS tests (executed by ptest-runner) contain a lengthly period of
silence while waiting for the DomUs to boot (>300s) so increase the
timeout.

Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>