Update nested dune test to emulate calling dune build more closely (#12916)

The way the test is implemented, the code testing dune is always calling
the (outside, Dune unter test) via its absolute path.

This change, extracted from #12902 calls Dune via absolute path but
wraps it in an utility where argv[0] is set to "dune" instead of the
absolute path, more closely resembling a call to `dune build` instead of
(a hypothetical) `/usr/bin/dune build`.

Author: Alizter

test/blackbox-tests/test-cases/pkg/different-dune-in-path.t

@@ -29,10 +29,6 @@ Make a project that depends on the test packages:
   >  (depends foo bar))
   > EOF
 
-  $ cat > dune <<EOF
-  > (data_only_dirs bin)
-  > EOF
-
 Make lockfiles for the packages.
   $ make_lockdir
   $ make_lockpkg foo <<EOF
@@ -65,18 +61,40 @@ Make lockfiles for the packages.
 Test that the project can be built normally.
   $ build_pkg foo
 
+
 Make a fake dune exe:
-  $ mkdir bin
-  $ cat > bin/dune <<EOF
+
+  $ mkdir .bin
+  $ cat > .bin/dune << 'EOF'
   > #!/bin/sh
-  > echo "Fake dune! (args: \$@)"
+  > echo "Fake dune! (args: $@)"
   > EOF
-  $ chmod a+x bin/dune
+  $ chmod a+x .bin/dune
 
   $ dune clean
+
 Try building in an environment where `dune` refers to the fake dune.
-  $ DUNE=$(which dune)  # otherwise we would start by running the wrong dune
-  $ PATH=$PWD/bin:$PATH $DUNE build $pkg_root/$($dune pkg print-digest foo)/target/
+
+Remember the path to the dune under test, otherwise we would launch the wrong
+Dune and add our fake `dune` to the PATH.
+
+  $ DUNE=$(which dune)
+  $ fakepath=$PWD/.bin:$PATH
+
+Remember the digests, to not to have to call nested Dunes:
+
+  $ foo_digest="$(dune pkg print-digest foo)"
+  $ bar_digest="$(dune pkg print-digest bar)"
+
+Call Dune with an absolute PATH as argv[0]:
+
+  $ PATH=$fakepath $DUNE build "$pkg_root/$foo_digest/target/"
   Fake dune! (args: build -p foo @install)
-  $ PATH=$PWD/bin:$PATH $DUNE build $pkg_root/$($dune pkg print-digest bar)/target/
+  $ PATH=$fakepath $DUNE build "$pkg_root/$bar_digest/target/"
   Fake dune! (args: build -p bar @install)
+
+Make sure that fake dune is not picked up when dune is called with argv[0] = "dune":
+
+  $ dune clean
+  $ PATH=$fakepath dune_cmd exec-a "dune" $DUNE build "$pkg_root/$foo_digest/target/"
+  Fake dune! (args: build -p foo @install)

test/blackbox-tests/utils/dune_cmd.ml

@@ -334,6 +334,29 @@ module Wait_for_file_to_appear = struct
   let () = register name of_args run
 end
 
+(* implements `exec -a` in a portable way *)
+module Exec_a = struct
+  type t =
+    { argv0 : string
+    ; prog : string
+    ; args : string list
+    }
+
+  let name = "exec-a"
+
+  let of_args = function
+    | argv0 :: prog :: args -> { argv0; prog; args }
+    | _ -> raise (Arg.Bad "Required arguments are <argv0> <program> [argument]...")
+  ;;
+
+  let run { argv0; prog; args } =
+    let args = Array.of_list @@ (argv0 :: args) in
+    Unix.execvp prog args
+  ;;
+
+  let () = register name of_args run
+end
+
 let () =
   let name, args =
     match Array.to_list Sys.argv with