testsuite - refactor test3.15 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 job memory. Use --wrap to avoid
creating a temporary job script.