make: add backend tag for itests

Defaults to running itests with btcd.

Makefile

@@ -115,7 +115,7 @@ scratch: build
 check: unit itest
 
 itest-only:
-	@$(call print, "Running integration tests.")
+	@$(call print, "Running integration tests with ${backend} backend.")
 	$(ITEST)
 
 itest: btcd build-itest itest-only
@@ -148,7 +148,7 @@ travis-itest: lint itest
 # =============
 
 flakehunter: build-itest
-	@$(call print, "Flake hunting integration tests.")
+	@$(call print, "Flake hunting ${backend} integration tests.")
 	while [ $$? -eq 0 ]; do $(ITEST); done
 
 flake-unit: