Ensure that perf regression tests cleanup properly

Each test in the performance regression test suite
creates a pool and a dataset for use. Unfortunately,
these tests do not cleanup the pool and dataset
correctly once they complete. Each test now kills
fio and iostat, destroys the dataset, and finally
destroys the pool. Each test also now traps the
SIGTERM signal to handle cases where test-runner
kills a test.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Requires-builders: all
Closes #5407