Fix the output of 'make bench' (ocaml-ppx#1877)

Author: gpetiot

Makefile

@@ -46,4 +46,6 @@ headers:
 
 .PHONY: bench
 bench:
-	@dune build @runbench
+	@dune build bench/test/source_bench.ml
+	@dune build bench/bench.exe
+	@dune exec bench/bench.exe

bench/bench.ml

@@ -24,7 +24,8 @@ type input =
   ; action: [`Format | `Numeric of range] }
 
 let inputs =
-  let source_ml = Stdio.In_channel.read_all "test/source_bench.ml" in
+  let dir = "_build/default/bench/test" in
+  let source_ml = Stdio.In_channel.read_all (dir ^ "/source_bench.ml") in
   [ { name= "format:conventional"
     ; input_name= "source.ml"
     ; kind= Syntax.Structure