LU-12795 tests: Prefer FAILF in mpi tests

LU-12795 tests: Prefer FAILF in mpi tests

Prefer FAILF() to the sprintf()/FAIL() pattern and remove
the errmsg temporary buffer.

Cleanup use of assignment in conditional statements.

Signed-off-by: Shaun Tancheff <stancheff@cray.com>
Change-Id: Icb04b188209ad3717260aa1238de1d9788d76f79
Reviewed-on: https://review.whamcloud.com/36276
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Petros Koutoupis <pkoutoupis@cray.com>
Reviewed-by: Arshad Hussain <arshad.super@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>