Improve coverage on AsyncHooksScopeManager (open-telemetry#178)

* fix: / update gitignore
.gitignore to ignore .nyc_output

* fix: increase feature coverage on ScopeManager

closes open-telemetry#163

* fix: use strictEqual()

* fix: typo

packages/opentelemetry-scope-async-hooks/test/asynchooks/AsyncHooksScopeManager.test.ts

@@ -63,6 +63,28 @@ describe('AsyncHooksScopeManager', () => {
         return done();
       });
     });
+
+    it('should rethrow errors', done => {
+      assert.throws(() => {
+        scopeManager.with(null, () => {
+          throw new Error('This should be rethrown');
+        });
+      });
+      return done();
+    });
+
+    it('should finally restore an old scope', done => {
+      const scope1 = 'scope1';
+      const scope2 = 'scope2';
+      scopeManager.with(scope1, () => {
+        assert.strictEqual(scopeManager.active(), 'scope1');
+        scopeManager.with(scope2, () => {
+          assert.strictEqual(scopeManager.active(), 'scope2');
+        });
+        assert.strictEqual(scopeManager.active(), 'scope1');
+        return done();
+      });
+    });
   });
 
   describe('.bind(function)', () => {