Fix docstring tests on Julia nightly / 1.11.

This addresses #523 by handling the new return type of `@doc` on Julia 1.11 nightly builds when the REPL module isn't loaded, which differs from the behavior of `@doc` on earlier versions of Julia.

test/dsl/dynamic_dsl.jl

@@ -534,16 +534,24 @@ end
 
 @testset "docstrings" begin
 
+    function doc_to_str(doc)
+        if doc isa Base.Docs.DocStr
+            # Handle @doc behavior in Julia 1.11 when REPL is not loaded
+            return doc.text[1]
+        else
+            # Handle pre-Julia 1.11 behavior of @doc
+            return strip(string(doc))
+        end
+    end
+
     """
     my documentation
     """
     @gen function foo(x)
             return x + 1
     end
 
-    io = IOBuffer()
-    print(io, @doc foo)
-    @test String(take!(io)) == "my documentation\n"
+    @test doc_to_str(@doc(foo)) == "my documentation"
 
 end