Previously practracker would fail on a file with no lines.

I'm using 500 as a file size limit, and 15 as an include limit.
This affects comparatively few files, but I think they are the worst
ones.

Closes ticket 31175.

This isn't actually something that Tor does, but it's cleaner to do
it this way.  Part of 29746.

The practracker_tests.py unit test file called a function by its old
name.

Also, practracker counted functions as starting one line after the
function name, and ending with the closing brace.  Now they start
with the open brace and end with the closing brace.

Previously we excluded any line containing one of these terms from
consideration as the start or end of a function.  Now we're more
careful, and we only ignore these terms when they appear to be
starting a function definition.

No longer messes up with MOCK_IMPL. Also update the tests to show that. We are
still being innacurate on the line count in some cases, but that doesnt matter
so much.