Fixed a data race bug in test codes

PiperOrigin-RevId: 528074883

internal/test/fake_clock.cc

@@ -21,6 +21,11 @@
 
 namespace nearby {
 
+FakeClock::~FakeClock() {
+  absl::MutexLock lock(&mutex_);
+  observers_.clear();
+}
+
 absl::Time FakeClock::Now() const {
   absl::MutexLock lock(&mutex_);
   return now_;

internal/test/fake_clock.h

@@ -33,6 +33,7 @@ class FakeClock : public Clock {
   FakeClock() { now_ = absl::Now(); }
   FakeClock(FakeClock&&) = default;
   FakeClock& operator=(FakeClock&&) = default;
+  ~FakeClock() override ABSL_LOCKS_EXCLUDED(mutex_);
 
   absl::Time Now() const override;