tests: remove `check` targets in subdirectories

Use a single `check` target at the `tests/` level like the project did
before to avoid stopping when a subdirectory fails and run all the
tests anyway. Use custom targets to run the tests of specific
directories (this only works from `tests/`) , for example:

    make check-cli check-plugins

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>