oeqa: make it work for multiple users

oeqa: make it work for multiple users

There are failures when multiple users run oe-selftest on the same
host:

PermissionError: [Errno 13] Permission denied: '/tmp/oe-saved-
tests/201812250324_qemu'

This is because /tmp/oe-saved-tests was created by user A, while user B tries
to write data in it, then the error will happen. This patch can fix the
problem.

Move the dumped data to ${LOG_DIR}/runtime-hostdump/ rather than
/tmp/oe-saved-tests/ to fix the problem.

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>