test: document -ccache option of idfx_test.py

Author: svalat

doc/source/testing/idfxTest.rst

@@ -74,6 +74,9 @@ The constructor parses command-line arguments using ``argparse``. These options
    * - ``-Werror``
      - ``Werror``
      - Treat compiler warnings as errors.
+   * - ``-ccache``
+     - ``ccache``
+     - Enable usage of ccache to build the tests and reduce the build time.
 
 Main Methods
 ------------