Update watch-assets to be less platform specific, change permissions in test-rerun

See discussion here https://yunity.slack.com/archives/C1T1ZM9A4/p1529764567000033

What does this MR do?

This MR does the following

• updates watch-assets to remove dependency on gnu readlink as it differs subtly across platforms.
• updates test-rerun to execute the removal of the downloads folder as root.

How confident are you it won't break things if deployed?

This needs to be tests across a few platforms to make sure I haven't broken anything.

Links to related issues

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• joined #foodsharing-beta channel at https://slackin.yunity.org
• added an entry to CHANGELOG.md (description, merge request link, username(s))