test: make stdout buffer test more robust
Checklist
-
tests and code linting passes -
the commit message follows commit guidelines
Affected core subsystem(s)
test
Description of change
test-stdout-buffer-flush-on-exit is unfortunately non-deterministic. It will, every so often, pass when it is supposed to fail. This is currently guarded against by running the test with three different long strings. This change increases it to five to reduce the false negatives.