Add a "many events" regression test.

Add a "many events" regression test.

This is a glass-box test to get more coverage on the event loop
backends.  We've run into bugs here before with fencepost errors, and
it turns out that none of our unit tests had enough events to
exercise the resize code.

Most of the backends have some kind of logic that resizes an array
when:
    - The highest fd is too high
    - The number of events added since the last iteration of the loop
      is too high
    - The number of active events is too high.

This test hits all 3 cases, and increases coverage in select.c by 7%,
in poll by 1%, and in kqueue by 9%.

svn:r1482