When executing multiple tests in parallel, each "child" thread would
write to the process's standard output buffer immediately upon test
completion. Because thread execution order and instruction interleaving
is non-deterministic, this made it possible for characters to be emitted
out-of-order.

When extended to support multiple concurrent threads, the runner was
outfitted with a "log lock" dedicated to sharing access to the output
file (when applicable). Re-use this lock when writing to standard out,
ensuring proper ordering of test result messages.