Add test for invalid virtual libraries

Specifically, the virtual module listed already has an implementation

Signed-off-by: Rudi Grinberg <rudi.grinberg@gmail.com>

src/dune_file.ml

@@ -853,7 +853,7 @@ module Library = struct
       let syntax =
         Syntax.create ~name:"in_development_do_not_use_variants"
           ~desc:"the experimental variants feature"
-          [ (0, 0) ]
+          [ (0, 1) ]
       in
       Dune_project.Extension.register syntax (Dsexp.Of_sexp.return []);
       syntax

test/blackbox-tests/test-cases/variants/invalid-virtual-lib/dune

@@ -0,0 +1,3 @@
+(library
+ (name foo)
+ (virtual_modules foo bar))

test/blackbox-tests/test-cases/variants/invalid-virtual-lib/dune-project

@@ -0,0 +1,3 @@
+(lang dune 1.1)
+
+(using in_development_do_not_use_variants 0.1)

test/blackbox-tests/test-cases/variants/run.t

@@ -1,16 +1,17 @@
 Variant feature is auto enabled when virtual_modules is used
 
   $ dune build --root variants-without-using
-  File "dune", line 3, characters 1-25:
-  3 |  (virtual_modules foobar))
-       ^^^^^^^^^^^^^^^^^^^^^^^^
-  Error: 'virtual_modules' is only available since version 0.1 of the experimental variants feature
-  [1]
+  Entering directory 'variants-without-using'
 
   $ dune build --root variants-using
-  File "dune-project", line 2, characters 42-45:
-  2 | 
-  Error: Version 0.1 of in_development_do_not_use_variants is not supported.
-  Supported versions:
-  - 0.0
+  Entering directory 'variants-using'
+
+virtual libraries may not implement their virtual modules
+
+  $ dune build --root invalid-virtual-lib
+  Entering directory 'invalid-virtual-lib'
+  File "dune", line 3, characters 18-21:
+  3 |  (virtual_modules foo bar))
+                        ^^^
+  Error: Module Foo has an implementation, it cannot be listed here
   [1]