builder: index_parser_tests: really use parameter

Really use the parameter of the "read_file" function, instead of
hardcoding "out".  This does not change the behaviour, since all the
callers already use "out" as the file name to read.

Fixes commit 7e3689bfe0.

builder/index_parser_tests.ml

@@ -46,7 +46,7 @@ let write_entries file entries =
   close_out chan
 
 let read_file file =
-  read_whole_file (tmpdir // "out")
+  read_whole_file (tmpdir // file)
 
 let parse_file file =
   let source = { Sources.name = "input";