-s should not cache output if tests are run serially

[ewu63]

Currently, -s will pipe the output directly to stdout, but I assume due to test concurrency the outputs are cached first and only shown after each test finishes, to avoid multiple outputs being jumbled. However, when the tests are run serially (with -n 1), they should be shown on screen immediately for faster monitoring and debugging.

Tagging @eirikurj who first brought up this point.