HttpClientTimeoutTests can occasionally fail due to unreachable network

[joakime]

Jetty version
9.4.x

Java version
11.0.7+

OS type/version
Linux

Description
The tests that utilize the IP 10.255.255.1 can sometimes result in an unexpected java.net.SocketException: Network is unreachable (connect failed) when the test is attempting to test for connect timeouts.

org.eclipse.jetty.http.client.HttpClientTimeoutTest.testConnectTimeoutIsCancelledByShorterRequestTimeout(Transport)[3]  Time elapsed: 0.006 s  <<< FAILURE!
org.opentest4j.AssertionFailedError: 
	at org.eclipse.jetty.http.client.HttpClientTimeoutTest.assumeConnectTimeout(HttpClientTimeoutTest.java:523)
	at org.eclipse.jetty.http.client.HttpClientTimeoutTest.testConnectTimeoutIsCancelledByShorterRequestTimeout(HttpClientTimeoutTest.java:345)
Caused by: java.net.SocketException: Network is unreachable (connect failed)
	at org.eclipse.jetty.http.client.HttpClientTimeoutTest.assumeConnectTimeout(HttpClientTimeoutTest.java:511)
	at org.eclipse.jetty.http.client.HttpClientTimeoutTest.testConnectTimeoutIsCancelledByShorterRequestTimeout(HttpClientTimeoutTest.java:345)

The method HttpClientTimeoutTest.assumeConnectTimeout() should be updated to handle this situation.

https://github.com/eclipse/jetty.project/blob/65de149f84b68e23fb2a2a849e80b06850c8cd29/tests/test-http-client-transport/src/test/java/org/eclipse/jetty/http/client/HttpClientTimeoutTest.java#L503-L525