test_mac_pkg.py: Fix the comments to describe what the script is doing.
test_mac_pkg.py: clean up the test code.

Instead of sleeping at the end of 'tahoe --version-and-path', we
now wait for the process to finish. As a consequence, we don't
need the code in the 'finally' block that kills the process.
Merge remote-tracking branch 'vu3rdd/182-osx-packaging-2' into 182-osx-packaging-3