tests: Fix the build with slibtool

When building the tests with `make check` and slibtool the tests will
then all fail to load libtpms.so.0.

  $ ./base64decode
  /tmp/libtpms/tests/.libs/base64decode: error while loading shared libraries: libtpms.so.0: cannot open shared object file: No such file or directory

This happens because they are linked with -ltpms rather than the
libtpms.la file which has unexpected results with slibtool. GNU libtool
does some magic to make this work while slibtool fails to link the
dependency.

The correct way to link internal dependencies is directly with the
libtool archive (.la) files where the -lfoo linker flags should be only
used with external dependencies. Additionally -no-undefined is added to
the LDFLAGS to ensure there aren't undefined references in the future.

Note:

* This doesn't happen if libtpms is installed to the system and the tests
  find the already installs libtpms rather than the newly built library.

* GNU libtool silently ignores -no-undefined, but slibtool will respect
  it.

Signed-off-by: orbea <orbea@riseup.net>

tests/Makefile.am

@@ -10,7 +10,8 @@ TESTS_ENVIRONMENT = \
   abs_top_srcdir=`cd '$(top_srcdir)'; pwd`
 
 AM_CFLAGS = -I$(top_srcdir)/include $(SANITIZERS)
-AM_LDFLAGS = -ltpms -L$(top_builddir)/src/.libs $(SANITIZERS)
+AM_LDFLAGS = -no-undefined $(SANITIZERS)
+LDADD = $(top_builddir)/src/libtpms.la
 
 check_PROGRAMS = \
 	base64decode