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

Signed-off-by: Vas Sudanagunta <vas@commonkarma.org>
PR-URL: #66563
Reviewed-By: Moshe Atlow <moshe@atlow.co.il>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

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
@@ -2025,6 +2035,7 @@ statement, since both are still in use while the event is being delivered; see
 [`database.close()`][] and [`statement.close()`][].
 
 [BoundedChannel Channels]: #boundedchannel-channels
+[Test instrumentation and OpenTelemetry]: test.md#test-instrumentation-and-opentelemetry
 [TracingChannel Channels]: #tracingchannel-channels
 [`'uncaughtException'`]: process.md#event-uncaughtexception
 [`--enable-fips-indicator-events`]: cli.md#--enable-fips-indicator-events