Make utils_test.py more robust.

Tkinter.Tk() requires $DISPLAY to be set or the
testCalculateVNCScreenRatio unittest fails. Replace the call with a set
screenname to avoid that issue.

Bug: None
Test: atest acloud_test (when $DISPLAY is unset)
Change-Id: I94aa13fec9e3071e83a41000bcce7c9cbcef974d
1 file changed