Fix ITSignatureTest.testSignPDF failure

03:17:33,148 ERROR [AbstractTest] Test 'org.nuxeo.ftest.signature.ITSignatureTest#testSignPDF' failed
org.openqa.selenium.TimeoutException: Timed out after 30 seconds waiting for org.nuxeo.functionaltests.Locator$2@6c37bd27
Build info: version: '2.53.0', revision: '35ae25b1534ae328c771e0856c93e187490ca824', time: '2016-03-15 10:43:46'
System info: host: 'itslave', ip: '172.17.0.2', os.name: 'Linux', os.arch: 'amd64', os.version: '4.4.0-24-generic', java.version: '1.8.0_77'
Driver info: driver.version: unknown
	at org.openqa.selenium.support.ui.FluentWait.timeoutException(FluentWait.java:292)
	at org.openqa.selenium.support.ui.FluentWait.until(FluentWait.java:261)
	at org.nuxeo.functionaltests.Locator.findElementsWithTimeout(Locator.java:131)
	at org.nuxeo.functionaltests.pages.AbstractPage.findElementsWithTimeout(AbstractPage.java:209)
	at org.nuxeo.functionaltests.pages.AbstractPage.getFeedbackMessage(AbstractPage.java:109)
	at org.nuxeo.functionaltests.pages.AbstractPage.getErrorFeedbackMessage(AbstractPage.java:92)
	at org.nuxeo.ftest.signature.ITSignatureTest.testSignPDF(ITSignatureTest.java:98)