Run dune fmt before dev/ test without failure and add make watch-test (…

…#1379)

Makefile

@@ -23,7 +23,8 @@ setup-instructor:
 setup-student: 
 	cp src/haz3lweb/ExerciseSettings_student.re src/haz3lweb/ExerciseSettings.re
 
-dev-helper: 
+dev-helper:
+	dune fmt --auto-promote || true
 	dune build @src/fmt --auto-promote src --profile dev
 
 dev: setup-instructor dev-helper
@@ -58,8 +59,12 @@ repl:
 	dune utop src/haz3lcore
 
 test:
+	dune fmt --auto-promote || true
 	dune build @src/fmt @test/fmt --auto-promote src test --profile dev
 	node $(TEST_DIR)/haz3ltest.bc.js
 
+watch-test:
+	dune build @fmt @runtest --auto-promote --watch
+
 clean:
 	dune clean