Add two child-process stdio flush tests
Checklist
-
make -j4 test
(UNIX) orvcbuild test nosign
(Windows) passes -
a test and/or benchmark is included -
the commit message follows commit guidelines
Affected core subsystem(s)
child-process
Description of change
Fixed a previous testcase that wasn't actually triggering the condition it tried to test. Added a new test that needs #7160 to be merged for it to pass.