runtime: call fflush before exiting in C test

Very, very rarely TestVectoredHandlerDontCrashOnLibrary fails because
the C subprocess exits with a 0 status code and no output. This
appears to happen because C does not actually guarantee that stdout
will be flushed on exit and somehow, very rarely, it is not flushed.

Add explicit fflushes to fix this. This reduces the failure rate of
TestVectoredHandlerDontCrashOnLibrary from 0.0013% to 0% in 250,000
iterations.

Fixes #49959.

Change-Id: I892cf49a165ac91134c5da37588a2ab11e1f3f8b
Reviewed-on: https://go-review.googlesource.com/c/go/+/380494


Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>