Add return code to the testpoint mechanism

The testpoint processing could fail and currently there is no mechanism
to notify the caller of such failures. This patch adds an int return
code to the testpoint prototype. Non-zero return code indicate failure.

When using the testpoint mechanism, the caller should properly handle
testpoint failure cases and trigger the appropriate response (error
handling, thread teardown, etc.).

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