Fix: src.ctf.fs: omitting to add ctf signature

Fix: src.ctf.fs: omitting to add ctf signature

When depacketizing a metadata stream we may need to add the CTF 1.8
signature at the beginning of the file if it's absent.

This fixes a regression introduces by commit:
  commit 06be99464988953e390114337ccdd20622fb50d1
  Author: Philippe Proulx <eeppeliteloop@gmail.com>
  Date:   Tue Jul 9 19:40:45 2019 -0400

      ctf: refactor metadata decoder to always have an instance

This regression was not caught earlier because the test case for that
feature was disabled.

This commit reintroduces that behaviour and enables the test case.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: I3621a7f8a21ffe51f29dfa344f30039fdf5d16c4
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8204
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
CI-Build: Simon Marchi <simon.marchi@efficios.com>