tests: context: Verify for out of bounds array

The for loop could exit with a out of bounds (variable j) value for
the delayed_threads array, we verify for the variable value before
operating on the array

This issue was reported by Coverity

Coverity-CID: 160078

Change-Id: I6aa1cc325cc363be48cd72b2a58d0a55ec3854bc
Signed-off-by: Sergio Rodriguez <sergio.sf.rodriguez@intel.com>