Prevent tests from hanging

When the NotebookApp raises an exception on initialization, a test can
hang forever. This can happen when passing configuration when
customizing NotebookTestBase:
```
class TestMissingExtension(NotebookTestBase):
    @classmethod
    def get_argv(cls):
        argv = super(TestMissingExtension, cls).get_argv()
        argv.extend(
            [
                "--NotebookApp.session_manager_class=doesnt_exist",
            ]
        )
        return argv

    def test_this_will_hang_forever(self):
        pass
```

Since the exception happens before the try/finally the `Event` will
never be triggered. By including the construction and initialization of
the notebook in the try/finally we can handle situations like this.

notebook/tests/launchnotebook.py

@@ -146,31 +146,31 @@ class NotebookTestBase(TestCase):
             if 'asyncio' in sys.modules:
                 import asyncio
                 asyncio.set_event_loop(asyncio.new_event_loop())
-            app = cls.notebook = NotebookApp(
-                port=cls.port,
-                port_retries=0,
-                open_browser=False,
-                config_dir=cls.config_dir,
-                data_dir=cls.data_dir,
-                runtime_dir=cls.runtime_dir,
-                notebook_dir=cls.notebook_dir,
-                base_url=cls.url_prefix,
-                config=config,
-                allow_root=True,
-                token=cls.token,
-            )
-            # don't register signal handler during tests
-            app.init_signal = lambda : None
-            # clear log handlers and propagate to root for nose to capture it
-            # needs to be redone after initialize, which reconfigures logging
-            app.log.propagate = True
-            app.log.handlers = []
-            app.initialize(argv=cls.get_argv())
-            app.log.propagate = True
-            app.log.handlers = []
-            loop = IOLoop.current()
-            loop.add_callback(started.set)
             try:
+                app = cls.notebook = NotebookApp(
+                    port=cls.port,
+                    port_retries=0,
+                    open_browser=False,
+                    config_dir=cls.config_dir,
+                    data_dir=cls.data_dir,
+                    runtime_dir=cls.runtime_dir,
+                    notebook_dir=cls.notebook_dir,
+                    base_url=cls.url_prefix,
+                    config=config,
+                    allow_root=True,
+                    token=cls.token,
+                )
+                # don't register signal handler during tests
+                app.init_signal = lambda : None
+                # clear log handlers and propagate to root for nose to capture it
+                # needs to be redone after initialize, which reconfigures logging
+                app.log.propagate = True
+                app.log.handlers = []
+                app.initialize(argv=cls.get_argv())
+                app.log.propagate = True
+                app.log.handlers = []
+                loop = IOLoop.current()
+                loop.add_callback(started.set)
                 app.start()
             finally:
                 # set the event, so failure to start doesn't cause a hang