tests: fail rather than infloop in tail's inotify-rotate test

tests: fail rather than infloop in tail's inotify-rotate test

* tests/tail-2/inotify-rotate: Switch to new init.sh-based framework.
(grep_timeout): New function.
Use it in place of open-coded loops that might infloop.
This was prompted by my encountering an inexplicable, and so far
unreproducible, infloop in the code that was waiting for "b" to
appear in "out".