Make stable link to latest execution report

[therealryan]

Execution reports are saved in a timestamped directory. While this is the safe position (execution reports are useful information that we shouldn't overwrite by default), it's kind of a pain to work with - iterating on a test will create a profusion of reports, so you might end up looking at a historic report rather than the latest one and it's not going to be particularly obvious from the URL.
You can change the destination directory via a system property to something stable, but then you're in the danger zone of overwriting reports.

A convenient middle ground would be to continue writing to timestamped directories, but to also create a symbolic link with a stable name to the latest report.

Bear in mind #495