shellutils: unit test stability improvement

• One of shellutils unit tests is grepping the output of the "ps" command, to look for the command "sleep" not to be in execution. This could have led to false positives, when the "sleep" command was maybe concurrently in execution because of system reasons (e.g. on SLC5 platforms). The test is now slightly more precise, by looking for a more exact string. (closes #1131)