Add info about test executor and how to use a custom executor

Author: aedart

docs/+current/modules/testing/cmake/05_executor.md

@@ -7,4 +7,33 @@ author: RSP Systems A/S
 
 # Test Executor
 
-_TODO: ..._
+When tests are registered (_via [ctest's `add_test()`](https://cmake.org/cmake/help/latest/command/add_test.html)_),
+a "test executor" (_cmake script_) is requested executed. The executor is then responsible for invoking the actual test
+callback, that has been specified via [`define_test()`](./03_test.md#parameters).
+In addition, the executor is also responsible for executing eventual
+[before and after callbacks](./02_test_case.md#before-after-callbacks), for the test case.
+
+## Location
+
+The executor can be found at:
+[`rsp/testing/executor.cmake`](https://github.com/rsps/cmake-scripts/blob/main/cmake/rsp/testing/executor.cmake).
+
+## Custom Executor
+
+To use a custom executor, set the path to your executor via the `RSP_TEST_EXECUTOR_PATH` property.
+This _SHOULD_ be done before specifying your test suites.
+
+```cmake
+# ...in your /tests/CMakeLists.txt
+
+enable_testing()
+
+project(my_package_tests LANGUAGES NONE)
+
+include("rsp/testing")
+
+# Set path to custom test executor
+set(RSP_TEST_EXECUTOR_PATH "../cmake/my_custom_test_executor.cmake")
+
+define_test_suite("unit" DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}/unit")
+```