Basic ProxyConfigurationManagerTest

[jglick]

Came across #8693 and noticed that the XML serial form of ProxyConfiguration was not tested. This is not trivial as can be seen from

jenkins/core/src/main/java/hudson/ProxyConfiguration.java

Lines 296 to 306 in 8b94924

	public static XmlFile getXmlFile() {
	return new XmlFile(XSTREAM, new File(Jenkins.get().getRootDir(), "proxy.xml"));
	}
	public static ProxyConfiguration load() throws IOException {
	XmlFile f = getXmlFile();
	if (f.exists())
	return (ProxyConfiguration) f.read();
	else
	return null;
	}

jenkins/core/src/main/java/hudson/ProxyConfiguration.java

Lines 506 to 512 in 8b94924

	private static final XStream XSTREAM = new XStream2();
	private static final long serialVersionUID = 1L;
	static {
	XSTREAM.alias("proxy", ProxyConfiguration.class);
	}

(There is also a getter & setter in Jenkins from #3935 which is tested downstream in configuration-as-code-plugin: ProxyConfiguratorTest.)

Testing done

Test passes locally.

Proposed changelog entries

• N/A

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[NotMyFault]

/label ready-for-merge

---

This PR is now ready for merge. We will merge it after ~24 hours if there is no negative feedback.
Please see the merge process documentation for more information about the merge process.
Thanks!