Add dummy match data

Apparently the abstract bug pops up again when there are no tests.

src/spectator/matchers/dummy_match_data.cr

@@ -0,0 +1,32 @@
+module Spectator::Matchers
+  # Match data that does nothing.
+  # This is to workaround a Crystal compiler bug.
+  # See: [Issue 4225](https://github.com/crystal-lang/crystal/issues/4225)
+  # If there are no concrete implementations of an abstract class,
+  # the compiler gives an error.
+  # The error indicates an abstract method is undefined.
+  # This class shouldn't be used, it's just to trick the compiler.
+  private struct DummyMatchData < MatchData
+    # Creates the match data.
+    def initialize
+      super(false)
+    end
+
+    # Dummy values.
+    def named_tuple
+      {
+        you: "shouldn't be calling this."
+      }
+    end
+
+    # Dummy message.
+    def message
+      "You shouldn't be calling this."
+    end
+
+    # Dummy message
+    def negated_message
+      "You shouldn't be calling this."
+    end
+  end
+end