MDL-80912 reportbuilder: output table caption when rendering reports.

Move the `get_name` method to the base report class, so that it can be
implemented by all report types. Provide a default implementation for
system reports based on the name of the class.

Use the name of the report as the caption. Target via Behat selectors.

reportbuilder/classes/datasource.php

@@ -47,13 +47,6 @@ abstract class datasource extends base {
     /** @var array $activeconditions */
     private $activeconditions;
 
-    /**
-     * Return user friendly name of the datasource
-     *
-     * @return string
-     */
-    abstract public static function get_name(): string;
-
     /**
      * Add columns from the given entity name to be available to use in a custom report
      *