stream test: make the sleep timeout configurable

Use the environment ALCOVE_TEST_STREAM_MAGIC_SLEEP to control the length
of time the shell process will wait before exiting. The default is 0
(no wait).

The underlying issue is that a parent process is exiting before relaying
all the stdout from the child shell process. While this problem needs
to be fixed, it is interrupting the stress tests which are turning up
some new, interesting failures.