Fixing constant cleaning when compiling tests

docs/testing_mbed_OS_5.md

@@ -10,6 +10,7 @@ The way tests are run and compiled in mbed OS 5 is substantially different from
     - [Test names](#test-names)
   - [Building tests](#building-tests)
     - [Building process](#building-process)
+    - [App config](#app-config)
   - [Running tests](#running-tests)
   - [Writing tests](#writing-tests)
 - [Debugging tests](#debugging-tests)
@@ -73,6 +74,12 @@ The full build process is:
 1. For each discovered test, build all of its source files and link it with the non-test code that was built in step 1.
 1. If specified, create a test specification file and place it in the given directory for use by testing tools. This is placed in the build directory by default when using mbed CLI.
 
+#### App config
+
+When building an mbed application, the presence of a `mbed_app.json` file allows you to set or override different config settings from libraries and targets. However, because the tests share a common build, this can cause issues when tests have different configurations that affect the OS.
+
+If you need to use app config, this must be set via the `--app-config` option when calling `mbed test`. **If this option is not specified, the build system will ignore all `mbed_app.json` files and use the default config values.**
+
 ### Running tests
 
 Automated tests can be run easily through mbed CLI. For information on using mbed CLI, please see its documentation.

tools/test.py

@@ -121,6 +121,15 @@
                                "Currently set search path: %s"
                        % (toolchain, search_path))
 
+        # App config
+        # Disable finding `mbed_app.json` files in the source tree if not
+        # explicitly defined on the command line. Config system searches for
+        # `mbed_app.json` files if `app_config` is None, but will set the
+        # app config data to an empty dictionary if the path value is another
+        # falsey value besides None.
+        if options.app_config is None:
+            options.app_config = ''
+
         # Find all tests in the relevant paths
         for path in all_paths:
             all_tests.update(find_tests(path, mcu, toolchain, options.options,
@@ -184,7 +193,6 @@
                                                 verbose=options.verbose,
                                                 notify=notify,
                                                 archive=False,
-                                                remove_config_header_file=True,
                                                 app_config=options.app_config)
 
                 library_build_success = True