Allow test fixtures and test methods to throw errors by default.

* test/testcase.vala (TestCase): Add a generic throws clause to both
  TestMethod and set_up and tear_down, update subclasses.

test/client/application/geary-configuration-test.vala

@@ -19,7 +19,7 @@ class ConfigurationTest : Gee.TestCase {
         this.test_config = new Configuration(GearyApplication.APP_ID);
     }
 
-    public void desktop_environment() {
+    public void desktop_environment() throws Error {
         assert(this.test_config.desktop_environment ==
                Configuration.DesktopEnvironment.UNKNOWN);