doc: add example of using filter with cctest

This commit adds examples of specifying filters when running cctest.
This can be useful when debugging and limiting the test cases run to
the test cases of interest and I think worth documenting.

PR-URL: #21401
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

doc/guides/writing-tests.md

@@ -403,6 +403,16 @@ The test can be executed by running the `cctest` target:
 $ make cctest
 ```
 
+A filter can be applied to run single/multiple test cases:
+```console
+$ make cctest GTEST_FILTER=EnvironmentTest.AtExitWithArgument
+```
+
+`cctest` can also be run directly which can be useful when debugging:
+```console
+$ out/Release/cctest --gtest_filter=EnvironmentTest.AtExit*
+```
+
 ### Node.js test fixture
 There is a [test fixture][] named `node_test_fixture.h` which can be included by
 unit tests. The fixture takes care of setting up the Node.js environment