Typed variables pt6: Clean up and test variable type guards (take 2) (#54025)

* Add fixture-factories for all variable types

* clean up variable guards

* add tests for isMulti, hasCurrent, and hasOptions variable type guards

* create VariableSupport type to try and work around ts wonkiness

packages/grafana-data/src/types/datasource.ts

@@ -173,6 +173,13 @@ export interface DataSourceConstructor<
   new (instanceSettings: DataSourceInstanceSettings<TOptions>, ...args: any[]): DSType;
 }
 
+// VariableSupport is hoisted up to its own type to fix the wonky intermittent
+// 'variables is references directly or indirectly' error
+type VariableSupport<TQuery extends DataQuery, TOptions extends DataSourceJsonData> =
+  | StandardVariableSupport<DataSourceApi<TQuery, TOptions>>
+  | CustomVariableSupport<DataSourceApi<TQuery, TOptions>>
+  | DataSourceVariableSupport<DataSourceApi<TQuery, TOptions>>;
+
 /**
  * The main data source abstraction interface, represents an instance of a data source
  *
@@ -341,10 +348,7 @@ abstract class DataSourceApi<
    * Defines new variable support
    * @alpha -- experimental
    */
-  variables?:
-    | StandardVariableSupport<DataSourceApi<TQuery, TOptions>>
-    | CustomVariableSupport<DataSourceApi<TQuery, TOptions>>
-    | DataSourceVariableSupport<DataSourceApi<TQuery, TOptions>>;
+  variables?: VariableSupport<TQuery, TOptions>;
 
   /*
    * Optionally, use this method to set default values for a query