lib/filter/tests: preload the syslogformat module

When linking the filter test program, preload the syslogformat module,
so that make check will work without installing syslog-ng.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
Makefile.am: Add PRELOAD_SYSLOGFORMAT and other helpers

Instead of hardcoding the same dlpreopen at many different locations,
add a helper variable to the top-level Makefile.am, and use that
variable in all other places.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
When doing make check in a subdir, use tests from subdirs too

Teach the Mk/subdir.mk snippet to recurse into subdirs of subdirs when
doing make check. This is a bit of a nasty hack, but gets the job done.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>