MXS-2236 Add own long test and possibility to run tests under Valgrind
Long test executes INSERT queries, transactions, prepared statements in parallel to create weird load on Maxscale to catch crashes and leaks
Test is not included into ctest scope. Test should be executed manually. For BuildBot (and also for run_test.sh) 'test_set' should be set 'NAME# ./long_test'
Time to run test is defined by 'long_test_time' variable (in seconds)
Possibility to run Maxscale under Valgrind is also added. To run Maxscale under Vaslgrind 'use_valgrind=yes' variable have to be defined
