tests: build the helper program by default

Usually we may configure and make, then goto ./tests/qemu-iotest,
check. In this case an error will happen since helper program
was not built. This patch simply build it by default. A better way
may be introducing Makefile in ./tests/qemu-iotest, but it is more
complicate to handle out of tree case, and a bit overkill
for a single file now, we can do that when more files come.

Signed-off-by: Wenchao Xia <xiawenc@linux.vnet.ibm.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
