tests/src.ctf.fs: add function to compute trace path in test-fail.sh

tests/src.ctf.fs: add function to compute trace path in test-fail.sh

Add a function that returns a trace path given a trace name and a ctf
version.  This is useful for subsequent patches that add tests with CTF
2 traces.

Change-Id: Ibc81d150152cd47e7113ebbc94bfe6995773655e
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12749
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>