tests: Make sure config file is empty when required.

We need to have no config at all for this test so
make sure it is empty.

Reported-by: Anna Czarnowska <anna.czarnowska@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>

tests/11spare-migration

@@ -57,6 +57,7 @@ monitor(){
 test0()
 {
 dsc "Test 0: No config file, no spare should be moved"
+> $config
 setupdevs 0 0 1 $platform
 setupdevs 1 3 4 $platform
 monitor 0 1