Sort testcases to make build (more) reproducible

f.Scope.Objects is a map, so iteration order isn't defined. To fix this,
lets just sort the testcase list so that it's always consistent.

There are still some paths in the built go files, but this improves
repeatability using the same paths.

Test: Build twice, compare generated test.go files.
Change-Id: Ie3fc896f54314fee0e0e9ddc3a9e06ad2f72a78d