golang.org/x/tools/go/ssa update causes build to fail

Recent update to golang/tools changes behaviour of CreateTestMainPackage (see golang/tools@8e53eb9) which now accepts only one ssa.Package instead of a slice. Call should be changed to use a for range loop instead.

The experimental nature of go/ssa needs a lot of maintenance. An option is to consider vendoring an old version of the ssa package.