Tests: Fix nprocesses applications shutdown

The nprocesses test is using the SIGPIPE signal to shutdown the spawned
child applications. This is done in order to silence the shell message
such as this one:

"nprocesses/test_nprocesses: line 79: 26458 Terminated"

This mechanism is broken on systems where the SIGPIPE default handler is
overriden (thus no application shutdown occurs) in the executing
environment. This could lead to stale applications polluting the system
and causing heratic behavior in subsequent tests.

This commit add a loop and check that all the spawned applications have
been successfully killed.

Signed-off-by: Christian Babeux <christian.babeux@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>