This PR change the longtest labels for some of the tests. 
Tests like stressHistogram, stressHistoFit, stressRooFit, stressRooStats are essentials and are test suites made of several individual tests. 
Their running for every PR is essential. 
Since for these tests an interpreted and a no interpreted version exists, flag as longtest only the non interpreted version

Start flagging instead as longtest other minor tests in math which take some time (longer than few seconds) but they are not critical.