Add concurrency stress test to 'make check' test set
Add concurrency stress test to 'make check' test set

- Currently the stress test does not take too long time to run to be
  included in 'make check'. If the test is extended and becomes too slow
  a separate make target (like stresstest) should be added for running it.

vranki/libqtsparql,vranki/libqtsparql,vranki/libqtsparql,vranki/libqtsparql