allow the test to configure which diagram to dump

Makefile

@@ -8,5 +8,6 @@
 .diag.html:
 	(cat $<; echo; echo '<!-- Markdeep: --><style class="fallback">body{visibility:hidden;white-space:pre;font-family:monospace}</style><script src="markdeep.min.js" charset="utf-8"></script><script src="https://casual-effects.com/markdeep/latest/markdeep.min.js" charset="utf-8"></script><script>window.alreadyProcessedMarkdeep||(document.body.style.visibility="visible")</script>') > $@
 
-diagtest:
-	ls box.sh diag.getxt Makefile | entr sh -c 'make diag.diag && cat diag.diag'
+DIAG?=diag
+test:
+	ls box.sh $(DIAG).getxt Makefile | entr sh -c 'make $(DIAG).diag && cat $(DIAG).diag'