Improve searching for TestingBuildParams using the output file

When searching for an TestingBuildParams this change will compare the
supplied path against the value returned by PathRelativeToTop() as well
as the String() and Rel() values. In the event that it cannot find the
file it also formats the available outputs, one per line with both rel
and path relative to top.

Bug: 182885307
Test: m nothing
Change-Id: Icbd37018b9a028abf37e27d1bea12a407316e031