docs: add no-op test targets for reference manuals

docs/reference/gdk/Makefile.am

@@ -154,3 +154,9 @@ include $(top_srcdir)/gtk-doc.make
 EXTRA_DIST += version.xml.in
 
 -include $(top_srcdir)/git.mk
+
+# This directory has no runtime tests.  Keep recursive `make test`
+# working when the top-level test target descends into the documentation
+# reference directories.
+test:
+

docs/reference/gtk/Makefile.am

@@ -386,3 +386,9 @@ dist-hook-local: $(BUILT_EXTRA_DIST)
 	  cp $$d/$$f $(distdir) || exit 1; done
 
 -include $(top_srcdir)/git.mk
+
+# This directory has no runtime tests.  Keep recursive `make test`
+# working when the top-level test target descends into the documentation
+# reference directories.
+test:
+

docs/reference/libgail-util/Makefile.am

@@ -34,3 +34,9 @@ HTML_IMAGES =
 include $(top_srcdir)/gtk-doc.make
 
 -include $(top_srcdir)/git.mk
+
+# This directory has no runtime tests.  Keep recursive `make test`
+# working when the top-level test target descends into the documentation
+# reference directories.
+test:
+