Add reproduction case for #5264

Signed-off-by: Jeremie Dimino <jeremie@dimino.org>

test/blackbox-tests/test-cases/github5264.t

@@ -0,0 +1,48 @@
+Reproduction case for #5264
+
+Before Dune 3.0, it was possible to abuse the
+%{lib:<libname>:<filename>} syntax to refer to the library directory
+by writing %{lib:<libname:} or %{lib:<libname>:.}. Several projects
+used that in their ctypes rules. Dune 3.0 initially broke this, which
+caused many build failures. This test makes sure we still support this
+syntax in lang Dune < 3.0.
+
+  $ cat > dune <<'EOF'
+  > (library (name ctypes))
+  > (rule
+  >  (alias a)
+  >  (action (run echo %{lib:ctypes:})))
+  > (rule
+  >  (alias b)
+  >  (action (run echo %{lib:ctypes:})))
+  > EOF
+
+At the moment, we get a bad error with all language versions:
+
+  $ echo '(lang dune 2.9)' > dune-project
+  $ dune build @a
+  Error: File unavailable: /home/dim/.opam/4.13.1/lib/ctypes
+  This is not a regular file (S_DIR)
+  -> required by %{lib:ctypes:} at dune:4
+  -> required by alias a in dune:2
+  [1]
+  $ dune build @b
+  Error: File unavailable: /home/dim/.opam/4.13.1/lib/ctypes
+  This is not a regular file (S_DIR)
+  -> required by %{lib:ctypes:} at dune:7
+  -> required by alias b in dune:5
+  [1]
+
+  $ echo '(lang dune 3.0)' > dune-project
+  $ dune build @a
+  Error: File unavailable: /home/dim/.opam/4.13.1/lib/ctypes
+  This is not a regular file (S_DIR)
+  -> required by %{lib:ctypes:} at dune:4
+  -> required by alias a in dune:2
+  [1]
+  $ dune build @b
+  Error: File unavailable: /home/dim/.opam/4.13.1/lib/ctypes
+  This is not a regular file (S_DIR)
+  -> required by %{lib:ctypes:} at dune:7
+  -> required by alias b in dune:5
+  [1]