use defer instead of os.Exit(m.Run())

as of go1.15 `testing.M` returns the return code of `m.Run()`, so it is
possible to use `defer`
golang/go#34129

README.md

@@ -100,14 +100,15 @@ func TestMain(m *testing.M) {
 		log.Fatalf("Could not connect to database: %s", err)
 	}
 
-	code := m.Run()
+	// as of go1.15 testing.M returns the exit code of m.Run(), so it is safe to use defer here
+    defer func() {
+      if err := pool.Purge(resource); err != nil {
+        log.Fatalf("Could not purge resource: %s", err)
+      }
 
-	// You can't defer this because os.Exit doesn't care for defer
-	if err := pool.Purge(resource); err != nil {
-		log.Fatalf("Could not purge resource: %s", err)
-	}
+    }()
 
-	os.Exit(code)
+	m.Run()
 }
 
 func TestSomething(t *testing.T) {

examples/FakeGoogleCloudStorage.md

@@ -63,8 +63,7 @@ func TestMain(m *testing.M) {
 		log.Fatalf("Could not connect to Docker: %s", err)
 	}
 
-	code := m.Run()
-	os.Exit(code)
+	m.Run()
 }
 
 func setUpGcloud() {

examples/MongoDB.md

@@ -66,20 +66,20 @@ func TestMain(m *testing.M) {
 		log.Fatalf("Could not connect to docker: %s", err)
 	}
 
-	// run tests
-	code := m.Run()
-
-	// When you're done, kill and remove the container
-	if err = pool.Purge(resource); err != nil {
-		log.Fatalf("Could not purge resource: %s", err)
-	}
+	defer func() {
+		// When you're done, kill and remove the container
+		if err = pool.Purge(resource); err != nil {
+			log.Fatalf("Could not purge resource: %s", err)
+		}
 
-	// disconnect mongodb client
-	if err = dbClient.Disconnect(context.TODO()); err != nil {
-		panic(err)
-	}
+		// disconnect mongodb client
+		if err = dbClient.Disconnect(context.TODO()); err != nil {
+			panic(err)
+		}
+	}()
 
-	os.Exit(code)
+	// run tests
+	m.Run()
 }
 
 ```

examples/Mountebank.md

@@ -78,15 +78,15 @@ func TestMain(m *testing.M) {
 	}); err != nil {
 		log.Fatalf("Could not connect to docker: %s", err)
 	}
-	//Run tests
-	code := m.Run()
 
-	// You can't defer this because os.Exit doesn't care for defer
-	if err := pool.Purge(resource); err != nil {
-		log.Fatalf("Could not purge resource: %s", err)
-	}
+	defer func() {
+		if err := pool.Purge(resource); err != nil {
+			log.Fatalf("Could not purge resource: %s", err)
+		}
+    }()
 
-	os.Exit(code)
+	// run tests
+	m.Run()
 }
 
 func TestHandler(t *testing.T) {

examples/PostgreSQL.md

@@ -65,15 +65,15 @@ func TestMain(m *testing.M) {
 	}); err != nil {
 		log.Fatalf("Could not connect to docker: %s", err)
 	}
-	//Run tests
-	code := m.Run()
 
-	// You can't defer this because os.Exit doesn't care for defer
-	if err := pool.Purge(resource); err != nil {
-		log.Fatalf("Could not purge resource: %s", err)
-	}
+	defer func() {
+		if err := pool.Purge(resource); err != nil {
+			log.Fatalf("Could not purge resource: %s", err)
+		}
+	}()
 
-	os.Exit(code)
+	// run tests
+	m.Run()
 }
 
 func TestRealbob(t *testing.T) {