Automatic merge from submit-queue

test/images/mount-tester: ensure exec binary is o+rx

The `mount-tester` image is currently used as a base layer for other
test images (like `mounttest-user`) which perform uid/gid changes.
However, the binary built in here just follows local umask, and currently is
```
-rwxr-x--- 1 root root 2052704 May 19  2016 mt
```

This commit adds an explicit chmod on the binary to make sure uid/gid
tests can still run it as "others".