.gitignore: cleanup test files

.gitignore: cleanup test files

Given that, under the `tests` directory:

✔ All shell file names end with `.sh`.
✔ All C file names end with `.c` or `.h`.
✔ All C++ file names end with `.cpp` or `.hpp`.
✔ No Python file name contains `-`.

Then it's safe to ignore everything named `test-` except the files of
which the name ends with one of the extensions above.

Also sort the list of remaining specific file names.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I2a4d541bd9624ed48051c944789c0b5f075a917b
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12466
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>