Add tests

Signed-off-by: nojebar <nicolas.ojeda.bar@lexifi.com>

test/blackbox-tests/test-cases/alternative-dune-fname.t/run.t

@@ -0,0 +1,43 @@
+Check support for alternative dune file name "dune-file".
+
+The feature is not supported in < 3.0.
+
+  $ cat >dune-project <<EOF
+  > (lang dune 2.8)
+  > (accept_alternative_dune_file_name)
+  > EOF
+  $ dune build
+  File "dune-project", line 2, characters 0-35:
+  2 | (accept_alternative_dune_file_name)
+      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  Error: 'accept_alternative_dune_file_name' is only available since version
+  3.0 of the dune language. Please update your dune-project file to have (lang
+  dune 3.0).
+  [1]
+
+The feature *is* supported in 3.0 and later, but it is opt-in:
+
+  $ cat >dune-project <<EOF
+  > (lang dune 3.0)
+  > EOF
+
+  $ cat >dune <<EOF
+  > (rule (alias foo) (action (echo "In dune")))
+  > EOF
+
+  $ cat >dune-file <<EOF
+  > (rule (alias foo) (action (echo "In dune-file")))
+  > EOF
+
+  $ dune build @foo
+  In dune
+
+Need to enable explicitly:
+
+  $ cat >dune-project <<EOF
+  > (lang dune 3.0)
+  > (accept_alternative_dune_file_name)
+  > EOF
+
+  $ dune build @foo
+  In dune-file