[lldbsuite] Fix the filecheck functionality to work with Python 3

Summary: This is another string/byte conversion issue between Python 2 and 3. In Python 2, the subprocess communication expects a byte string, but in Python 3, it expects bytes. Since both versions are capable of using strings when universal_newlines is set to True AND filecheck operates on strings, force the use of strings.

Reviewers: zturner, asmith, vsk

Reviewed By: zturner

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D53166

llvm-svn: 344386