Makefile.am: Use the -no-install libtool flag for tests

When building test programs (anything in ${check_PROGRAMS}) add
-no-install to the list of LDFLAGS, so that libtool stops generating a
useless wrapper for them. This makes debugging the check programs
easier, and also avoids a lot of shell launching for no good reason.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>