doc: adds "type" to test runner event details

In version 20.0.0 (and backported to 19.9.0 and 18.7.0) the test runner
started reporting on whether a test was a suite. This was exposed to
reporters in the `details` object of a `test:pass` or `test:fail` event
but this hasn't been documented. This adds the `type` property to both
event's `details` object.

doc/api/test.md

@@ -2061,6 +2061,8 @@ Emitted when a test is enqueued for execution.
     * `duration_ms` {number} The duration of the test in milliseconds.
     * `error` {Error} An error wrapping the error thrown by the test.
       * `cause` {Error} The actual error thrown by the test.
+    * `type` {string|undefined} The type of the test, used to denote whether
+      this is a suite
   * `file` {string|undefined} The path of the test file,
     `undefined` if test was run through the REPL.
   * `name` {string} The test name.
@@ -2076,6 +2078,8 @@ Emitted when a test fails.
 * `data` {Object}
   * `details` {Object} Additional execution metadata.
     * `duration_ms` {number} The duration of the test in milliseconds.
+    * `type` {string|undefined} The type of the test, used to denote whether
+      this is a suite
   * `file` {string|undefined} The path of the test file,
     `undefined` if test was run through the REPL.
   * `name` {string} The test name.