testsuite - refactor test2.8 to avoid memory enforcement.

Commit 63b7e3a8 changed the --mem limit to 1MB for the step if
not using a memory SelectType, but this can cause the job to fail
if the JobAcctGatherFrequency is frequent enough to notice that the
"sleep" command is using more than 1MB of resources.

Refactor test to avoid specifying memory memory; and since only one
step is checked for, only run a single step in the job. Use --wrap
to avoid creating a temporary job script.