Ensure Virtual tables can be declared outside rusqlite crate

Not sure it is the way to go.

Cargo.toml

@@ -58,6 +58,9 @@ harness = false
 [[test]]
 name = "deny_single_threaded_sqlite_config"
 
+[[test]]
+name = "vtab"
+
 [package.metadata.docs.rs]
 features = [ "backup", "blob", "chrono", "functions", "limits", "load_extension", "serde_json", "trace" ]
 all-features = false