ensure lldbinline remembers .py extension

This ensure lldbinline.test_file paths are tracked as .py
files rather than .pyc files.

Also, this change adds an assert to the test infrastructure
if a filename that is not ending in .py is attempted to be
added to the test events infrastructure where we track test
results.

See:
http://reviews.llvm.org/D19215

Earlier revision reviewed by:
Pavel Labath

llvm-svn: 266664