Fix duplicate calls for oneTimeTearDown()

Additional oneTimeTearDown() was called at shunit2 exit because there
seemed to be no set for __shunit_clean flag. Fix for kward#112

Author: cezanne

shunit2

@@ -1367,6 +1367,7 @@ _shunit_execSuite
 if ! oneTimeTearDown; then
   _shunit_fatal "oneTimeTearDown() returned non-zero return code."
 fi
+__shunit_clean=${SHUNIT_TRUE}
 
 # Generate a report summary.
 _shunit_generateReport