Skip tests that are slow on 32-bit builders

zpool_create_024_pos, zvol_misc_002_pos, write_dirs_002_pos are slow
on the buildbot 32-bit builder. Skip the test cases for now on 32-bit
builders.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Closes #6195