[test-suite] Signal error if llvm-lit was not found

Summary:
I'm not sure if that was intentional or not?
I found it quite surprising when trying to integrate a new external project.

Reviewers: MatzeB, homerdin, hfinkel

Reviewed By: hfinkel

Subscribers: mgorny, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60091

llvm-svn: 364388
1 file changed