Fix failing test (change the way reference to TestHttpClient is obtained...

Spec fails due to missing method exception on `LocalScriptApplicationUnderTest#httpClient()`. Getting the `TestHttpClient` via `TestHttpClients.testHttpClient(aut)` fixes the problem.

In addition, the "captures output" spec fails on Windows due to the use of Unix-specific line separator (\n). Denormalising the corresponding GString (with Groovy's `denormalize()` method) fixes the problem.