Add zpool_destroy_004_pos test case.

This test targets the ability of 'zpool destroy -f' to tear down the pool
even if there is activity on it. In particular, this tests a background zfs
send | zfs receive.

zpool_destroy_004_pos tests the following combinations:

• Data size of 1GB: sleep times 0.1 0.3 0.5 0.75 1 2 3 The goal is to test the race conditions at various points during the process of 'zfs send' and 'zfs receive', with respect to when the 'zpool destroy -f' takes place.
• Data size of 4/3 physical memory: sleep time 15 The goal is to validate that the destroy process teardown time is orthagonal to the send stream size.

Each test point above looks like this:

for sleeptime in <sleep times>; do
  Run the tests where the sender and receiver are the same pool
  Run the tests where they differ; destroy the sender
  Run the tests where they differ; destroy the receiver
done

The tests in each step above perform the following:

• Create the sender pool (and receiver pool if applicable)
• Create a dataset on the sender, populate it, then snapshot it
• Start the send | receive in the background
• Sleep $sleeptime seconds
• Do the forced destroy of the target pool, must succeed
• Wait for the background subshell to exit non-zero
• Verify that the target pool is indeed no longer visible

Submitted by: Will
Sponsored by: Spectra Logic Corp