Add null constructor

src/spectator/context_delegate.cr

@@ -1,5 +1,6 @@
 require "./context"
 require "./context_method"
+require "./null_context"
 
 module Spectator
   # Stores a test context and a method to call within it.
@@ -10,6 +11,14 @@ module Spectator
     def initialize(@context : Context, @method : ContextMethod)
     end
 
+    # Creates a delegate with a null context.
+    # The context will be ignored and the block will be executed in its original scope.
+    def self.null(&block : -> _)
+      context = NullContext.new
+      method = ContextMethod.new { block.call }
+      new(context, method)
+    end
+
     # Invokes a method in the test context.
     def call
       @method.call(@context)