Let some tests skip themselves which are sensitive to the live contents of the update center

[jglick]

On a Javadoc-only PR (#3958) I got four test failures which were apparently due to some transient problem in the update center (or at least I cannot reproduce the failures locally). Two representative examples:

java.lang.NullPointerException
	at hudson.model.UpdateCenter2Test.install(UpdateCenter2Test.java:51)

net.sf.json.JSONException: Expected a ',' or '}' at character 99537 of {"connectionCheckUrl":"http://www.google.com/",…,"name":"audit-trail",…,"wiki":"https://plugins.jenkins.io/audit-trail"}
	at net.sf.json.util.JSONTokener.syntaxError(JSONTokener.java:499)
	at net.sf.json.JSONObject._fromJSONTokener(JSONObject.java:1043)
	at net.sf.json.JSONObject.fromObject(JSONObject.java:156)
	at net.sf.json.util.JSONTokener.nextValue(JSONTokener.java:348)
	at net.sf.json.JSONObject._fromJSONTokener(JSONObject.java:955)
	at net.sf.json.JSONObject._fromString(JSONObject.java:1145)
	at net.sf.json.JSONObject.fromObject(JSONObject.java:162)
	at net.sf.json.JSONObject.fromObject(JSONObject.java:132)
	at hudson.model.UpdateSite.updateData(UpdateSite.java:205)
	at hudson.model.UpdateSite.updateDirectlyNow(UpdateSite.java:188)
	at hudson.model.UpdateCenterPluginInstallTest.setup(UpdateCenterPluginInstallTest.java:48)
	at hudson.model.UpdateCenterPluginInstallTest.test_installUnknownPlugin(UpdateCenterPluginInstallTest.java:55)

[batmat]