feat(testing): Promisify the bootstrap function

Make sure the bootstrap test setup function
returns a `Promise` (instead of `(done) => void`).

BREAKING CHANGE: `boostrap(MyServer)` now returns a `Promise`.

Change
```typescript
beforeEach(done => bootstrap(MyServer)(done))
```
to
```typescript
beforeEach(async () => await bootstrap(MyServer)())
```
or
```typescript
beforeEach(bootstrap(MyServer))
```

packages/testing/src/TestContext.ts

@@ -1,11 +1,15 @@
 import {InjectorService} from "@tsed/di";
 
 export class TestContext {
-  static injector: InjectorService | null;
+  static injector: InjectorService | null = null;
+
+  /**
+   * Resets the test injector of the test context, so it won't pollute your next test. Call this in your `tearDown` logic.
+   */
   static reset() {
-    if (this.injector) {
-      this.injector.clear();
+    if (TestContext.injector && TestContext.injector.clear) {
+      TestContext.injector.clear();
     }
-    this.injector = null;
+    TestContext.injector = null;
   }
 }

packages/testing/src/bootstrap.ts

@@ -1,28 +1,24 @@
 import {$log} from "ts-log-debug";
-import {TestContext} from "./testContext";
+import {TestContext} from "./TestContext";
 
 /**
  * Load the server silently without listening port and configure it on test profile.
  * @decorator
  * @param server
  * @param args
- * @returns {(done:Function)=>undefined}
+ * @returns {Promise<void>}
  */
-export function bootstrap(server: any, ...args: any[]) {
+export function bootstrap(server: any, ...args: any[]): () => Promise<void> {
   $log.stop();
-  process.env.NODE_ENV = process.env.NODE_ENV || "test";
 
-  return function before(done: any) {
+  return function before() {
     const instance = new server(...args);
 
     instance.startServers = () => Promise.resolve();
 
     // used by inject method
     TestContext.injector = instance.injector;
 
-    instance
-      .start()
-      .then(done)
-      .catch(done);
+    return instance.start();
   };
 }

packages/testing/src/index.ts

@@ -3,3 +3,4 @@ export * from "./done";
 export * from "./bootstrap";
 export * from "./invoke";
 export * from "./loadInjector";
+export * from "./TestContext";

packages/testing/src/inject.ts

@@ -1,6 +1,6 @@
 import {Done} from "./done";
 import {loadInjector} from "./loadInjector";
-import {TestContext} from "./testContext";
+import {TestContext} from "./TestContext";
 
 /**
  * The inject function is one of the TsExpressDecorator testing utilities.