mark Coq 8.9 as broken

Makefile.coq.local

@@ -8,6 +8,7 @@ test: $(TESTFILES:.v=.vo)
 .PHONY: test
 
 COQ_TEST=$(COQTOP) $(COQDEBUG) -batch -test-mode
+COQ_BROKEN=$(shell echo "$(COQ_VERSION)" | egrep "^8\.9\b" > /dev/null && echo 1)
 
 # Can't use pipes because that discards error codes and dash provides no way to control that.
 # Also egrep errors if it doesn't match anything, we have to ignore that.
@@ -20,13 +21,17 @@ tests/.coqdeps.d: $(TESTFILES)
 -include tests/.coqdeps.d
 
 $(TESTFILES:.v=.vo): %.vo: %.v $(if $(MAKE_REF),,%.ref)
-	$(SHOW)$(if $(MAKE_REF),COQTEST [ref],COQTEST) $<
+	$(SHOW)$(if $(MAKE_REF),COQTEST [ref],$(if $(COQ_BROKEN),COQTEST [ignored],COQTEST)) $<
 	$(HIDE)TEST="$$(basename -s .v $<)" && \
 	  TMPFILE="$$(mktemp)" && \
 	  $(TIMER) $(COQ_TEST) $(COQFLAGS) $(COQLIBS) -load-vernac-source $< > "$$TMPFILE" && \
 	  ($(REF_FILTER) < "$$TMPFILE" > "$$TMPFILE.filtered" || true) && \
 	  $(if $(MAKE_REF), \
 	    mv "$$TMPFILE.filtered" "tests/$$TEST.ref", \
-	    diff -u "tests/$$TEST.ref" "$$TMPFILE.filtered") && \
+	    $(if $(COQ_BROKEN), \
+	      true, \
+	      diff -u "tests/$$TEST.ref" "$$TMPFILE.filtered" \
+	    ) \
+	  ) && \
 	  rm -f "$$TMPFILE" "$$TMPFILE.filtered" && \
 	  touch $@