Add test to verify C++-to-Python LLSD notation sequence.

Write a sequence of LLSDSerialize::toNotation() calls separated by newlines to
a data file, then read lines and parse using llbase.llsd.parse(). Verify that
this produces expected data even when one item is a string containing newlines.
Generalize python() helper function to allow using any of the NamedTempFile
constructor forms.
Allow specifying expected Python rc (default 0) and use this to verify an
intentional sys.exit(17). This is better than previous sys.exit(0) test
because when, at one point, NamedTempFile failed to write file data, running
Python on an empty script file still terminates with rc 0. A nonzero rc
verifies that we've written the file, that Python is running it and that we're
retrieving its rc.