Update "tests/README.md"

This patch adds detail section on adding and running
test-case. It also changes markdown number list to
more readeable headers

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Signed-off-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Closes #12737