LU-13960 tests: correct usage of _var variable

LU-13960 tests: correct usage of _var variable

In the setmodopts() function in functions.sh, the '_var'
variable is set and used.  There is one use of the variable
'var' which should be '_var'.  Change the use of 'var' to
'_var'.

Test-Parameters: trivial
Test-Parameters: testlist=conf-sanity env=ONLY=53 clientdistro=ubuntu2004 fstype=ldiskfs
Test-Parameters: testlist=conf-sanity env=ONLY=53 clientdistro=ubuntu1804 fstype=ldiskfs
Test-Parameters: testlist=conf-sanity env=ONLY=53 clientdistro=el7.8 fstype=ldiskfs
Signed-off-by: James Nunez <jnunez@whamcloud.com>
Change-Id: If524be1f0b4b2170a514a558256a5308c9a5e586
Reviewed-on: https://review.whamcloud.com/39891
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>
Reviewed-by: Vikentsi Lapa <vlapa@whamcloud.com>