Add messages to assertFalse/True to clarify the failed assertion

Otherwise the output would just read "Failed asserting that true is
false." or "Failed asserting that false is true.", which is not very
informative when there are several assertFalse/True in a row.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>

tests/acceptance/features/bootstrap/PublicShareContext.php

@@ -202,11 +202,14 @@ class PublicShareContext implements Context, ActorAwareInterface {
 		// download item should not be shown in the menu (although it will be in
 		// the DOM).
 		PHPUnit_Framework_Assert::assertFalse(
-				$this->actor->find(self::downloadItemInShareMenu())->isVisible());
+				$this->actor->find(self::downloadItemInShareMenu())->isVisible(),
+				"Download item in share menu is visible");
 		PHPUnit_Framework_Assert::assertTrue(
-				$this->actor->find(self::directLinkItemInShareMenu())->isVisible());
+				$this->actor->find(self::directLinkItemInShareMenu())->isVisible(),
+				"Direct link item in share menu is not visible");
 		PHPUnit_Framework_Assert::assertTrue(
-				$this->actor->find(self::saveItemInShareMenu())->isVisible());
+				$this->actor->find(self::saveItemInShareMenu())->isVisible(),
+				"Save item in share menu is not visible");
 	}
 
 	/**