Update testing.md

Add updated information with active voice to App config section

Author: Amanda Butler

docs/advanced/testing.md

@@ -76,9 +76,11 @@ The full build process is:
 
 #### 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.
+When building an mbed application, the presence of an `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.**
+The build system will look for an `mbed_app.json` file in your shared project files (any directory not inside of a `TESTS` folder). If the system finds it, this configuration file will be used for both the nontest code as well as each test case inside your project's source tree. If there is more than one `mbed_app.json` file in the source tree, the config system will error.
+
+If you need to test with multiple configurations, you can use the `--app-config` option. This will override the search for an `mbed_app.json` file and use the config file you specify for the build.
 
 ### Running tests