build: Fix build of tests with non-default installation.

* tests/Makefile.am: Add forgotten @LDADD_FOR_TESTS_KLUDGE@.

Possible linking option by @LDADD_FOR_TESTS_KLUDGE@ matters.

When system has old libgcrypt and developer tests not-yet-installed
libgcrypt by make check, without this, because LD_LIBRARY_PATH has
precedence than DT_RUNPATH, those test programs will use old libgcrypt
under LD_LIBRARY_PATH.

• Fixes-commit: 9b8ac13761f0407bd701e43b0a65fbada204958f
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>