Extend MDCReactorSpec's http client read timeout (#10031)

This sets the read timeout of the main http client (which is manually
instantiated) used in MDCReactorSpec to 5 minutes, which is the same
as what is set for the dependency-injected clients used elsewhere
throughout the test.

This spec was frequently failing in CI with an HTTP read timeout error,
likely due to resource starvation on the CI job runner causing slow
responses from the embedded server that is used in the test.

context-propagation/src/test/groovy/io/micronaut/context/propagation/MDCReactorSpec.groovy

@@ -17,6 +17,7 @@ import io.micronaut.http.annotation.Header
 import io.micronaut.http.annotation.Patch
 import io.micronaut.http.annotation.Post
 import io.micronaut.http.annotation.Put
+import io.micronaut.http.client.DefaultHttpClientConfiguration
 import io.micronaut.http.client.HttpClient
 import io.micronaut.http.client.annotation.Client
 import io.micronaut.http.filter.ClientFilterChain
@@ -55,7 +56,13 @@ class MDCReactorSpec extends Specification {
 
     @Shared
     @AutoCleanup
-    HttpClient client = HttpClient.create(embeddedServer.URL)
+    HttpClient client
+
+    def setup() {
+        def config = new DefaultHttpClientConfiguration()
+        config.setReadTimeout(Duration.ofMinutes(5))
+        client = HttpClient.create(embeddedServer.URL, config)
+    }
 
     void "test MDC propagates"() {
         expect: