Tests: Fix missing TAP output in overlap test

The start and stop tracing shell functions are currently redirected to
/dev/null. This has the unfortunate side-effect of silencing the TAP
output thus producing an incomplete test output.

This commit remove the /dev/null redirection of those commands.

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