Make FixtureLogObserver thread-safe

We can log for different threads (main, map, workers, etc) so
we need to lock before adding the messages on the list.

This was causing few tests to randomly crash.

test/fixtures/fixture_log_observer.cpp

@@ -31,16 +31,22 @@ bool FixtureLog::Observer::onRecord(EventSeverity severity,
                                     Event event,
                                     int64_t code,
                                     const std::string& msg) {
+    std::lock_guard<std::mutex> lock(messagesMutex);
+
     messages.emplace_back(severity, event, code, msg);
 
     return true;
 }
 
 bool FixtureLog::Observer::empty() const {
+    std::lock_guard<std::mutex> lock(messagesMutex);
+
     return messages.empty();
 }
 
 size_t FixtureLog::Observer::count(const Message& message) const {
+    std::lock_guard<std::mutex> lock(messagesMutex);
+
     size_t message_count = 0;
     for (const auto& msg : messages) {
         if (msg == message) {
@@ -70,6 +76,8 @@ FixtureLog::~FixtureLog() {
 }
 
 std::vector<FixtureLog::Message> FixtureLogObserver::unchecked() const {
+    std::lock_guard<std::mutex> lock(messagesMutex);
+
     std::vector<Message> unchecked_messages;
     for (const auto& msg : messages) {
         if (!msg.checked) {

test/fixtures/fixture_log_observer.hpp

@@ -5,6 +5,7 @@
 
 #include <vector>
 #include <cstdarg>
+#include <mutex>
 #include <iostream>
 
 namespace mbgl {
@@ -42,9 +43,10 @@ public:
         size_t count(const Message& message) const;
         std::vector<Message> unchecked() const;
 
-    public:
+    private:
         FixtureLog* log;
         std::vector<Message> messages;
+        mutable std::mutex messagesMutex;
     };
 
     FixtureLog();