Add a delay between tests to make them more robust

Bug: 154342874
Test: atest VtsHalBiometricsFaceV1_0TargetTest
Change-Id: I09713b6814ecb9f4ee4a7f568b1f891a94f4e017
diff --git a/biometrics/face/1.0/vts/functional/VtsHalBiometricsFaceV1_0TargetTest.cpp b/biometrics/face/1.0/vts/functional/VtsHalBiometricsFaceV1_0TargetTest.cpp
index 7ac44a4..78f93af 100644
--- a/biometrics/face/1.0/vts/functional/VtsHalBiometricsFaceV1_0TargetTest.cpp
+++ b/biometrics/face/1.0/vts/functional/VtsHalBiometricsFaceV1_0TargetTest.cpp
@@ -28,6 +28,7 @@
 #include <chrono>
 #include <cstdint>
 #include <random>
+#include <thread>
 
 using android::sp;
 using android::hardware::hidl_vec;
@@ -144,7 +145,10 @@
         ASSERT_EQ(Status::OK, static_cast<Status>(ret2));
     }
 
-    void TearDown() override {}
+    void TearDown() override {
+        // Hack to allow the asynchronous operations to finish on time.
+        std::this_thread::sleep_for(std::chrono::milliseconds(250));
+    }
 
     sp<IBiometricsFace> mService;
     sp<FaceCallback> mCallback;