Fix LDFLAGS for tests

Unlike Makefile.am, which does not overwrite LDFLAGS but defines binary-specific
LDFLAGS variables, tests/Makefile.am redefines LDFLAGS and overwrites an
already existing LDFLAGS variable.

To successfully pass this variable to the test binaries, it needs to be appended
to the newly defined LDFLAGS variable.

tests/Makefile.am

@@ -7,6 +7,6 @@ LOG_DRIVER = env AM_TAP_AWK='$(AWK)' $(SHELL) \
 check_PROGRAMS = t_fticks t_rewrite t_resizeattr t_rewrite_config
 AM_CFLAGS = -g -Wall -Werror @SSL_CFLAGS@ @TARGET_CFLAGS@
 LDADD = $(top_builddir)/librsp.a @SSL_LIBS@
-LDFLAGS = @SSL_LDFLAGS@ @TARGET_LDFLAGS@
+LDFLAGS = @SSL_LDFLAGS@ @TARGET_LDFLAGS@ @LDFLAGS@
 
 TESTS = $(check_PROGRAMS)