qemu-iotests: export TEST_DIR for non-bash tests

Since qemu-iotests may need to create large image files it is possible
to specify the test directory.  The TEST_DIR variable needs to be
exported so non-bash tests can make use of it.
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

tests/qemu-iotests/common.config

@@ -121,6 +121,8 @@ if [ ! -d "$TEST_DIR" ]; then
     exit 1
 fi
 
+export TEST_DIR
+
 _readlink()
 {
     if [ $# -ne 1 ]; then