FileSystemPermissionTest: assert /system mounted RO

Make sure the /system partition is mounted read-only.
Leaving it read-write makes it easier for someone to
attack the device.

Credit:
* http://www.androidpolice.com/2013/01/28/exclusive-one-click-root-for-the-republic-wireless-motorola-defy-xt-single-and-dual-band-with-the-newest-ota-1_65k_1027-or-older/
* https://github.com/CunningLogic/Cyanide/blob/master/src/com/cunninglogic/cyanide/MainActivity.java

Change-Id: I3fe4d764387eb6fdc9fd789899104b2d5361ca04