Update nested dune test to emulate dune build closer

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 ocaml#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`.

Signed-off-by: Marek Kubica <marek@tarides.com>

Author: Leonidas-from-XIV

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

@@ -25,12 +25,26 @@ Make a project that depends on the test packages:
   > (lang dune 3.13)
   > (package
   >  (name x)
-  >  (allow_empty)
   >  (depends foo bar))
   > EOF
 
   $ cat > dune <<EOF
-  > (data_only_dirs bin)
+  > (executable
+  >  (name exec_a)
+  >  (public_name exec-a)
+  >  (libraries unix))
+  > EOF
+
+Small helper binary to implement `exec -a` in a portable way
+
+  $ cat > exec_a.ml <<EOF
+  > let () =
+  >   let desired_argv0 = Sys.argv.(1) in
+  >   let prog = Sys.argv.(2) in
+  >   let args = Array.length Sys.argv in
+  >   let new_args = Array.init (args - 2) (fun i -> Sys.argv.(i+2)) in
+  >   new_args.(0) <- desired_argv0;
+  >   Unix.execvp prog new_args
   > EOF
 
 Make lockfiles for the packages.
@@ -65,18 +79,54 @@ 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: \$@)"
   > EOF
-  $ chmod a+x bin/dune
+  $ chmod a+x .bin/dune
+
+  $ dune build @install
+  $ mkdir .temp
+  $ dune install --prefix $PWD/.temp
+  $ cp .temp/bin/exec-a $PWD/.bin/
+
+Show that the binary works like `exec -a` would work
+
+  $ .bin/exec-a "desired-argv0" sh -c 'echo $0'
+  desired-argv0
 
   $ dune clean
+
 Try building in an environment where `dune` refers to the fake dune.
+
+Remember the path to the dune under test:
+
   $ 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 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=$PWD/.bin:$PATH $DUNE build "$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=$PWD/.bin:$PATH $DUNE build "$pkg_root/$bar_digest/target/"
   Fake dune! (args: build -p bar @install)
+
+Call Dune with argv[0] set to a relative PATH. Make sure "dune" in PATH refers
+to the fake dune:
+
+  $ (PATH=$PWD/.bin:$PATH exec-a "dune" sh -c "which dune")
+  $TESTCASE_ROOT/.bin/dune
+
+Make sure that fake dune is not picked up when dune is called with argv[0] = "dune":
+
+  $ dune clean
+  $ (PATH=$PWD/.bin:$PATH exec-a "dune" $DUNE build "$pkg_root/$foo_digest/target/")
+  Fake dune! (args: build -p foo @install)