Fix color @usernames sometimes not working at all (#3170)

Definitely memory fuckery involved - The comment from @lubieerror https://github.com/Chatterino/chatterino2/issues/2822#issuecomment-897252673 is finally what led me to adding tests and hopefully fixing this.

src/common/ChannelChatters.cpp

@@ -74,6 +74,12 @@ void ChannelChatters::updateOnlineChatters(
     chatters_->updateOnlineChatters(chatters);
 }
 
+size_t ChannelChatters::colorsSize() const
+{
+    auto size = this->chatterColors_.access()->size();
+    return size;
+}
+
 const QColor ChannelChatters::getUserColor(const QString &user)
 {
     const auto chatterColors = this->chatterColors_.access();

src/common/ChannelChatters.hpp

@@ -25,9 +25,13 @@ public:
     void setUserColor(const QString &user, const QColor &color);
     void updateOnlineChatters(const std::unordered_set<QString> &chatters);
 
-private:
+    // colorsSize returns the amount of colors stored in `chatterColors_`
+    // NOTE: This function is only meant to be used in tests and benchmarks
+    size_t colorsSize() const;
+
     static constexpr int maxChatterColorCount = 5000;
 
+private:
     Channel &channel_;
 
     // maps 2 char prefix to set of names