TestFileHandle.py: fix for Python 3.6

Summary:
Python 3.6 stringifies exceptions as `ExceptionClass("foo",)` instead
of `ExceptionClass("foo")`.   This patch makes the test assertions a
little more flexible so the test passes anyway.

Reviewers: JDevlieghere, jasonmolenda, labath

Reviewed By: labath

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D68745

llvm-svn: 374417