runtests: add option -u to error on server unexpectedly alive

runtests: add option -u to error on server unexpectedly alive

Let's try to actually handle the server unexpectedly alive
case by first making them visible on CI builds as failures.

This is needed to detect issues with killing of the test
servers completely including nested process chains with
multiple PIDs per test server (including bash and perl).

On Windows/cygwin platforms this is especially helpful with
debugging PID mixups due to cygwin using its own PID space.

Reviewed-by: Daniel Stenberg
Closes #7180