[TestCompletion] Spawn LLDB with '--no-use-colors'.

Summary:
TestCompletion was broken for Ubuntu (and probably for Debian also).
The issue was that the lldb prompt in color (which is the default
behavior) was confusing pexpect.

Test Plan: "http://199.223.233.39:8011/lldb-try-clang-build-clang-test/"

Reviewers: zturner, clayborg

Reviewed By: clayborg

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D6671

llvm-svn: 224642