unit-tests: Enable libstrongswan tests even if --enable-unit-tests not set

As we don't depend on the check framework anymore, we can enable the unit tests
by default. These are built/executed with "make check" only, so it makes no
sense to disable them.

src/libstrongswan/Makefile.am

@@ -488,9 +488,7 @@ if MONOLITHIC
 endif
 endif
 
-if UNITTESTS
 if MONOLITHIC
   SUBDIRS += .
 endif
-  SUBDIRS += tests
-endif
+SUBDIRS += tests