test: add for opam switch name with absolute path (#10730)

* test: add a test for #10721

Signed-off-by: Etienne Marais <dev@maiste.fr>

* Update test/blackbox-tests/test-cases/github10721.t

Signed-off-by: Etienne Millon <etienne.millon@gmail.com>

---------

Signed-off-by: Etienne Marais <dev@maiste.fr>
Signed-off-by: Etienne Millon <etienne.millon@gmail.com>
Co-authored-by: Etienne Millon <etienne.millon@gmail.com>

test/blackbox-tests/test-cases/github10721.t

@@ -0,0 +1,18 @@
+  $ cat > dune-project << EOF
+  > (lang dune 3.0)
+  > EOF
+
+  $ cat > dune-workspace << EOF
+  > (lang dune 3.0)
+  > (context
+  >  (opam
+  >   (switch /absolute/path/to/opam/switch)))
+  > EOF
+
+  $ dune build
+  File "dune-workspace", line 4, characters 10-39:
+  4 |   (switch /absolute/path/to/opam/switch)))
+                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  Error: Generated context name "/absolute/path/to/opam/switch" is invalid
+  Please specify a context name manually with the (name ..) field
+  [1]