iotests: Fix 232 for LUKS

With IMGOPTSSYNTAX, $TEST_IMG is useless for this test (it only tests
the file-posix protocol driver).  Therefore, if $TEST_IMG_FILE is set,
use that instead.

Because this test requires the file protocol, $TEST_IMG_FILE will always
be set if $IMGOPTSSYNTAX is true.
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Reviewed-by: NJohn Snow <jsnow@redhat.com>
Message-id: 20190210145736.1486-5-mreitz@redhat.com
Signed-off-by: NMax Reitz <mreitz@redhat.com>

tests/qemu-iotests/232

@@ -69,6 +69,10 @@ size=128M
 
 _make_test_img $size
 
+if [ -n "$TEST_IMG_FILE" ]; then
+    TEST_IMG=$TEST_IMG_FILE
+fi
+
 echo
 echo "=== -drive with read-write image: read-only/auto-read-only combinations ==="
 echo