Mock out image source file in image_utils tests

Mock out image source file in image_utils tests

Change 202ebc2d987c86984ba9cf8e8e36c32899baaa3b
added some metrics gathering around time to fetch
and convert images.  To do this it uses os.stat,
and when it was written and tested on Linux everything
was cool.

Turns out however there were a few more methods that needed
mocks for the stat command.  This patch adds them.

Also, it was pointed out that for this specific set of tests
we don't actually need to set the class stat_results, mock
will just work for us here since we're not checking an explicit
value.  So this patch also removes those class decl's from
previous methods as they're not needed and we should keep
things consistent.

Change-Id: I31e85694bd167c08a585594b2675dc7232429e05