test-release.sh: Defer test errors until the end

This makes the script run to the end and produce tarballs even on test
failures, and then highlights any errors afterwards.

(I first tried just storing the errors in a global variable, but that
didn't work as the "test_llvmCore" function invocation is actually
running as a sub-shell.)

Differential Revision: http://reviews.llvm.org/D11478

llvm-svn: 243116