Clerk tests: use separate dirs for different flags, to preserve caching

The default output remains at the top of _build if no specific flags have been
given, the others are in `_build/test<flags>`

Makefile

@@ -203,12 +203,12 @@ unit-tests: .FORCE
 	dune build @for-tests @runtest
 
 #> test					: Run interpreter tests
-test: .FORCE
+test: .FORCE compiler
 	$(CLERK_TEST)
 
 tests: test
 
-TEST_FLAGS_LIST = \
+TEST_FLAGS_LIST = ""\
 -O \
 --lcalc \
 --lcalc,--avoid-exceptions,-O

build_system/clerk_driver.ml

@@ -868,6 +868,11 @@ let build_statements include_dirs dir =
 
 let gen_ninja_file catala_exe catala_flags build_dir include_dirs test_flags dir
     =
+  let build_dir =
+    match test_flags with
+    | [] -> build_dir
+    | flags -> File.((build_dir / "test") ^ String.concat "" flags)
+  in
   let ( @+ ) = Seq.append in
   Seq.return
     (Nj.Comment (Printf.sprintf "File generated by Clerk v.%s\n" version))