LU-14473 test: check RUNAS and RUNAS_ID

LU-14473 test: check RUNAS and RUNAS_ID

Validate RUNAS and RUNAS_ID before testing a file create, so
that the error messages can be more specific.

Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Change-Id: I87b2c279f981b34ab979cca42a8ae06128a294cc
Reviewed-on: https://review.whamcloud.com/41785
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Gian-Carlo DeFazio <defazio1@llnl.gov>
Reviewed-by: Oleg Drokin <green@whamcloud.com>