Improve copy_files test for c files to make sure source is tracked

test/blackbox-tests/test-cases/copy_files/jbuild

@@ -12,3 +12,8 @@
 (executable
  ((name test)
   (libraries (foo))))
+
+(alias
+ ((name bar-source)
+  (deps (bar.h))
+  (action (echo "${read:bar.h}"))))

test/blackbox-tests/test-cases/copy_files/run.t

@@ -10,3 +10,6 @@
     ocamlmklib dllfoo_stubs.so,libfoo_stubs.a
       ocamlopt test.{cmx,o}
       ocamlopt test.exe
+  $ $JBUILDER build -j1 @bar-source --root .
+  #line 1 "include/bar.h"
+  int foo () {return 42;}