Add documentation for injecting an HttpClient

See micronaut-projects/micronaut-test#716

src/main/docs/guide/testing.adoc

@@ -0,0 +1,23 @@
+When you create a Micronaut Framework application (either via https://launch.micronaut.io or the command line application), it creates a https://docs.micronaut.io/latest/api/io/micronaut/context/annotation/ContextConfigurer.html[ContextConfigurer] with enables https://docs.micronaut.io/latest/guide/index.html#eagerInit[eager singleton initialization].
+
+As tests annotated with `@MicronautTest` are implicitly in the `Singleton` scope, this can cause problems injecting some beans (for example an `HttpClient`) into your test class.
+
+To avoid this, you can either disable eager singleton initialization for your tests, or you will need to manually get an instance of the bean you would normally inject.  As an example, to get an `HttpClient` you could do:
+
+[source,java]
+----
+@Inject
+EmbeddedServer server; // <1>
+
+Supplier<HttpClient> client = SupplierUtil.memoizedNonEmpty(() ->
+    server.getApplicationContext().createBean(HttpClient.class, server.getURL())); // <2>
+
+@Test
+void testEagerSingleton() {
+    Assertions.assertEquals("eager", client.get().toBlocking().retrieve("/eager")); // <3>
+}
+----
+
+<1> Inject the `EmbeddedServer` as normal
+<2> Create a `Supplier` that will create the `HttpClient` when it is first called
+<3> Use the `Supplier` to get the `HttpClient` and make the request

src/main/docs/guide/toc.yml

@@ -7,5 +7,6 @@ refresh: Refresh scope
 events: Events
 context: Context Provider
 docker: Docker Support
+testing: Testing
 repository: Repository