Add test illustrating #6575 (#6576)

Signed-off-by: Ulysse Gérard <[email protected]>

test/blackbox-tests/test-cases/github6575.t

@@ -0,0 +1,24 @@
+Given a working directory cram test:
+  $ mkdir sometest.t
+  $ cat >sometest.t/run.t <<EOF
+  >   $ echo "foobar"
+  >   foobar
+  > EOF
+
+  $ dune build @sometest
+
+We turn it into a single-file test:
+  $ mv sometest.t sometest.t.bak
+  $ mv sometest.t.bak/run.t sometest.t
+  $ rm -r sometest.t.bak
+
+FIXME: Dune should detect the change:
+  $ dune build @sometest
+  Error: _build/default/sometest.t: Is a directory
+  -> required by _build/default/sometest.t
+  -> required by alias sometest
+  [1]
+
+After a clean it works as expected:
+  $ dune clean
+  $ dune build @sometest