Refactor checksum operations in tests

md5sum in particular but also sha256sum to a lesser extent is used
in several areas of the test suite for computing checksums. The vast
majority of invocations are followed by | awk '{ print $1 }'.

Introduce functions to wrap up md5sum $file | awk '{ print $1 }' and
likewise for sha256sum. These also serve as a convenient interface for
alternative implementations on other platforms.

Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@ixsystems.com>
Closes #9280