Inject Micronaut Http client at integrationstests

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### Task List

- [x] Steps to reproduce provided
- [n/a] Stacktrace (if present) provided
- [n/a] Example that reproduces the problem uploaded to Github
- [x] Full description of the issue provided (see below)

### Steps to Reproduce
When doing automatic integration testing in Micronaut, I can inject the embedded httpServer without specifying host nor port. This is very convenient as the port is randomised by default in tests.

Here is an example Spock testcase from Micronaut, where I have a HelloController that returns "Hello World":
```
@Inject
@Client('/') // <-- no need to specify port nor host
RxHttpClient client

void "call app"() {
when:
String reply = client.toBlocking().retrieve('/hello')

then:
reply == 'Hello World'
}
```

With Micronaut HTTP client being introduced in Grails 4.0, it would be very nice if we could avoid having to specify the hostname and port for Micronaut HTTP client in Grails integrationtests as well.

Right now I am doing this to get the Micronaut HTTP client in my integrationtests in Grails:
```
@LocalServerPort
Integer localServerPort // random port in tests

BlockingHttpClient httpClient

def setup() {
httpClient = httpClient ?: HttpClient.create(new URL("http://localhost:${localServerPort}")).toBlocking()
}
```

It would be very nice if I could do like in Micronaut:
```
@Inject
@Client('/') // <-- no need to specify port nor host
RxHttpClient client
```
### Expected Behaviour
the http client should be injected without needing to specify port and host

### Actual Behaviour
the http client does not get injected

### Environment Information

- **Operating System**: Ubuntu 18.04
- **Grails Version:** 4.0.0
- **JDK Version:** 8u212
- **Container Version (If Applicable):** N/A

### Example Application

- N/A - it is a feature request

まず、Issue に記載されている Grails の統合テスト設定と Micronaut の @Inject/@Client の配線を見つけます。組み込みサーバーのランダムポートがどのように公開されるかを確認し、その後、統合テストでホストやポートを指定せずに HTTP クライアントをインジェクトできることを検証します。

索引モデルが issue の本文から書いたものです。