tests: clean up test_trimmer

tests: clean up test_trimmer

This patch changes test_trimmer to reduce duplication of the
boilerplate.  I intend to add some test cases to this test in a
following patch, so this change allows me to understand better what the
test does currently and will help adding test cases.

Note that there are pairs of tests that are pretty much redundant, such
as:

- "Read a trace with the trimmer enabled (GMT relative timestamps)"
- "Ran successfully with --begin and --end (GMT relative timestamps)"

So I have only kept the second test of such pairs.

Change-Id: I075df21ed9c03495ab516d3870ccf7b63276d3ce
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1407
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>