Add some more filter tests (#4897)

This changes the `make coverage` function to use `gcovr` instead of `lcov`, and to have it generate an html file directly at `coverage/index.html` under the build directory

The only thing this changes, other than adding tests, is making the `Expression` class pure virtual. Every derived class should implement each of the functions

src/controllers/filters/lang/expressions/Expression.cpp

@@ -2,24 +2,4 @@
 
 namespace chatterino::filters {
 
-QVariant Expression::execute(const ContextMap & /*context*/) const
-{
-    return false;
-}
-
-PossibleType Expression::synthesizeType(const TypingContext & /*context*/) const
-{
-    return IllTyped{this, "Not implemented"};
-}
-
-QString Expression::debug(const TypingContext & /*context*/) const
-{
-    return "";
-}
-
-QString Expression::filterString() const
-{
-    return "";
-}
-
 }  // namespace chatterino::filters

src/controllers/filters/lang/expressions/Expression.hpp

@@ -16,10 +16,10 @@ class Expression
 public:
     virtual ~Expression() = default;
 
-    virtual QVariant execute(const ContextMap &context) const;
-    virtual PossibleType synthesizeType(const TypingContext &context) const;
-    virtual QString debug(const TypingContext &context) const;
-    virtual QString filterString() const;
+    virtual QVariant execute(const ContextMap &context) const = 0;
+    virtual PossibleType synthesizeType(const TypingContext &context) const = 0;
+    virtual QString debug(const TypingContext &context) const = 0;
+    virtual QString filterString() const = 0;
 };
 
 using ExpressionPtr = std::unique_ptr<Expression>;