tests/plugins/ctf/Makefile.am: do not set `TESTS` in this file

tests/plugins/ctf/Makefile.am: do not set `TESTS` in this file

We want the project's `make check` to continue even if specific tests
fail, so we can't define `TESTS` in a subdirectory's `Makefile.am`,
because if the test fails, `make` fails, and subsequent tests are not
executed.

There are ways to circumvent this, for example using `make`'s
`--always-make` option, but it was decided previously that we want
`make check` to run all tests as is, without any option.

Therefore we set `TESTS` to all tests in `tests/Makefile.am`.

To execute tests specific to a subdirectory, we have the following
targets:

* `check-bindings`
* `check-cli`
* `check-lib`
* `check-plugins`
* `check-python-plugin-provider`

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I1f198ff3ff2b44947042aee7fa5f01759874941f
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1254
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>