Add support for ports in withTestApplication

Currently there is no parameter for which port to use in sending a test request, I propose to add this function:
```kotlin
fun TestApplicationEngine.handleRequest(
method: HttpMethod,
port: Int = 80,
uri: String,
setup: TestApplicationRequest.() -> Unit = {}
): TestApplicationCall = handleRequest {
this.uri = uri
this.method = method
this.addHeader(HttpHeaders.Host, "localhost:$port")
setup()
}
```

Start by locating TestApplicationEngine.handleRequest and the withTestApplication test helpers, then inspect existing request-handling tests. Add port support to the test request API as described and verify that the generated Host header uses the requested port.

Written by the indexing model from the issue text.