tests: fix tap.c use of uninitialized pipe_r_file

[ Edit: updated patch header ]

The _tap_comment_stdout thread can start using pipe_r_file when it
is still uninitialized. Fix it by moving the initialization before the pthread
creation.

Signed-off-by: Yannick Brosseau <yannick.brosseau@gmail.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>