Clear cache for runs of ctest if --cache not set.
Clear cache for runs of ctest if --cache not set.

Change-Id: I013c838c83dd23cbfe04f77f87df74778d8e874e

BUG=chromium-os:10723
TEST=Ran it

Review URL: http://codereview.chromium.org/6389004

smilart/scripts,marineam/coreos-scripts,mjg59/scripts,sigma/coreos-scripts,mjg59/scripts,bcwaldon/coreos-scripts,BugRoger/scripts,endocode/scripts,trnubo/scripts,trnubo/scripts,crawford/scripts,gtank/scripts,mischief/scripts,nicescale/scripts,BugRoger/scripts,sigma/coreos-scripts,gtank/scripts,BugRoger/scripts,andrejro/coreos--scripts,endocode/scripts,glevand/coreos--scripts,bcwaldon/coreos-scripts,gtank/scripts,mjg59/scripts,trnubo/scripts,endocode/scripts,crawford/scripts,zhang0137/scripts,sigma/coreos-scripts,coreos/scripts,mischief/scripts,smilart/scripts,zhang0137/scripts,nicescale/scripts,vmware/coreos-scripts,marineam/coreos-scripts,vmware/coreos-scripts,marineam/coreos-scripts,zhang0137/scripts,smilart/scripts,andrejro/coreos--scripts,nicescale/scripts,vmware/coreos-scripts,andrejro/coreos--scripts,crawford/scripts,fivethreeo/scripts,fivethreeo/scripts,glevand/coreos--scripts,fivethreeo/scripts,coreos/scripts,bcwaldon/coreos-scripts,mischief/scripts