docs: Add testing documentation (#210)

Add testing doc

developer/wip/for-adopters/edc-testing.md

@@ -0,0 +1,29 @@
+# Testing
+
+EDC provides a JUnit test fixture for running automated integration tests. The EDC JUnit runtime offers a number of advantages:
+
+- Fast build time since container images do not need to be built and deployed
+- Launch and debug tests directly within an IDE
+- Easily write asynchronous tests using libraries such as [Awaitility](http://www.awaitility.org/)
+
+The JUnit runtime can be configured to include custom extensions. Running multiple instances as part of a single test setup is also possible. The following demonstrates how to set up and launch a basic test using JUnit's `RegisterExtension` annotation and the `RuntimePerClassExtension`:
+
+```java
+@EndToEndTest
+class Basic01basicConnectorTest {
+
+    @RegisterExtension
+    static RuntimeExtension connector = new RuntimePerClassExtension(new EmbeddedRuntime(
+            "connector",
+            emptyMap(),
+            ":basic:basic-01-basic-connector"
+    ));
+
+    @Test
+    void shouldStartConnector() {
+        assertThat(connector.getService(Clock.class)).isNotNull();
+    }
+}
+```
+
+For more details and examples, check out the [EDC Samples system tests](https://github.com/eclipse-edc/Samples/tree/main/system-tests).