overlaytests: fix testrunner.py md5sum invocation

The overlaytests testrunner.py script wants to invoke adb shell md5sum
to compute the md5 of a file, but wrongly invokes it as md5.  Maybe
it used to have that name, but it doesn't exist under that name in master.
Fix it.

Change-Id: I3ec4be162fab0cd3915f010f9b1add37b5397973
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>