WIP Enable configuration of TOP_DIR

[gotcha]

used by default_filter_filename.

The test suite of my tool runs forked processes in a temporary directory (via tempfile.mkdtemp) which does not share the same root directory as the test suite.
This implies all calls are filtered out by default_filter_filename.

Adding the set_top_dir was enough to avoid filtering out.

This is a simple enough way to work around the little naive assumption made by:

TOP_DIR = os.path.join(os.getcwd(), '')     # current dir with trailing slash

Would you see a better solution ?

[gvanrossum]

I would put the function directly after the definition of the variables, so it's harder for a reader to miss that these are not really constants.

[gotcha]

I have moved the definition of the variables closer to where they are used.

See #112 to avoid global vars while still enabling top_dir customization.

[CLAassistant]

All committers have signed the CLA.