test: combine stdout/stderr from failed test
    
    Printing stdout and stderr from a failed test makes it harder to
    interpret what the specific problem was; instead let's print out
    the lines in order as we got them when the test was run
    
    Also save failed test output to file if ARTIFACT_DIRECTORY is defined
    
    (cherry picked from commit d57e871c60805e3f1f57729ea8022477a611697d)