Add locations-check test (#844)

* add locations-check test

* update tests

ppx/test/locations-check.t

@@ -0,0 +1,66 @@
+Test the preprocessed reason-react components have well-formed locations.
+Uses https://github.com/ocaml-ppx/ppxlib/blob/44583fc14c3cc39ee6269ffd69f52146283f72c0/src/location_check.mli
+
+With no annotations (ppx does nothing)
+
+  $ cat >input.ml <<EOF
+  > let make ~foo ~bar =
+  >   (div
+  >      ~children:[ React.string foo; bar |> string_of_int |> React.string ]
+  >      ())
+  > EOF
+
+  $ reason-react-ppx -check -locations-check input.ml
+  let make ~foo  ~bar  =
+    div ~children:[React.string foo; (bar |> string_of_int) |> React.string] ()
+
+With JSX annotation
+
+  $ cat >input.ml <<EOF
+  > let make ~foo ~bar =
+  >   (div
+  >      ~children:[ React.string foo; bar |> string_of_int |> React.string ]
+  >      () [@JSX])
+  > EOF
+
+  $ reason-react-ppx -check -locations-check input.ml
+  File "input.ml", line 2, characters 3-6:
+  2 |   (div
+         ^^^
+  Error: invalid output from ppx, expression overlaps with expression at location:
+  File "input.ml", line 2, characters 2-96:
+  [1]
+
+With @react.component annotation
+
+  $ cat >input.ml <<EOF
+  > let[@react.component] make ~foo ~bar =
+  >   (div
+  >      ~children:[ React.string foo; bar |> string_of_int |> React.string ]
+  >      () [@JSX])
+  > EOF
+
+  $ reason-react-ppx -check -locations-check input.ml
+  File "input.ml", line 1, characters 33-36:
+  1 | let[@react.component] make ~foo ~bar =
+                                       ^^^
+  Error: invalid output from ppx, core type overlaps with core type at location:
+  File "input.ml", line 1, characters 33-36:
+  [1]
+
+Everything
+
+  $ cat >input.ml <<EOF
+  > let[@react.component] make ~foo ~bar =
+  >   (div
+  >      ~children:[ React.string foo; bar |> string_of_int |> React.string ]
+  >      () [@JSX])
+  > EOF
+
+  $ reason-react-ppx -check -locations-check input.ml
+  File "input.ml", line 1, characters 33-36:
+  1 | let[@react.component] make ~foo ~bar =
+                                       ^^^
+  Error: invalid output from ppx, core type overlaps with core type at location:
+  File "input.ml", line 1, characters 33-36:
+  [1]