lit: print process output, if getting the list of google-tests failed.

Summary:
This is a follow up to r280455, where a check for the process exit code
was introduced. Some ASAN bots throw this error now, but it's impossible
to understand what's wrong with them, and the issue is not reproducible.

Reviewers: vitalybuka

Differential Revision: https://reviews.llvm.org/D24210

llvm-svn: 280550