docs: add Test Runner to list of built-in diagnostic channels

Signed-off-by: Vas Sudanagunta <vas@commonkarma.org>

Author: vassudanagunta

doc/api/diagnostics_channel.md

@@ -1930,6 +1930,16 @@ Emitted when [`child_process.spawn()`][] encounters an error.
 
 Emitted when [`process.execve()`][] is invoked.
 
+#### Test Runner
+
+<!-- YAML
+added:
+ - v26.1.0
+ - v24.16.0
+-->
+
+See [Test instrumentation and OpenTelemetry][] for details.
+
 #### Web Locks
 
 > Stability: 1 - Experimental
@@ -2054,6 +2064,7 @@ statement, since both are still in use while the event is being delivered; see
 [`process.execve()`]: process.md#processexecvefile-args-env
 [`start` event]: #startevent
 [`statement.close()`]: sqlite.md#statementclose
+[Test instrumentation and OpenTelemetry]: test.md#test-instrumentation-and-opentelemetry
 [`worker_threads.locks`]: worker_threads.md#worker_threadslocks
 [context loss]: async_context.md#troubleshooting-context-loss
 [thenable object]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise#thenables