Update known issues to use new mock system

spec/issues/github_issue_28_spec.cr

@@ -7,10 +7,10 @@ Spectator.describe "GitHub Issue #28" do
     end
   end
 
-  # mock Test
+  mock Test
 
-  xit "matches method stubs with no_args", pending: "Mock redesign" do
-    test = Test.new
+  it "matches method stubs with no_args" do
+    test = mock(Test)
     expect(test).to receive(:foo).with(no_args).and_return(42)
     test.foo
   end

spec/issues/github_issue_33_spec.cr

@@ -10,13 +10,13 @@ Spectator.describe "GitHub Issue #33" do
     end
   end
 
-  # mock Test do
-  #   stub method2
-  # end
+  mock Test
 
   describe Test do
+    subject { mock(Test) }
+
     describe "#method1" do
-      xit pending: "Mock redesign" do
+      it do
         expect(subject).to receive(:method2)
 
         subject.method1

spec/issues/github_issue_42_spec.cr

@@ -25,13 +25,11 @@ class Sdk < SdkInterface
 end
 
 Spectator.describe Example do
-  # mock Sdk do
-  #   stub register_hook(name, &block)
-  # end
+  mock Sdk
 
   describe "#configure" do
-    xit "registers a block on configure", pending: "Mock redesign" do
-      sdk = Sdk.new
+    it "registers a block on configure" do
+      sdk = mock(Sdk)
       example_class = Example.new(sdk)
       allow(sdk).to receive(register_hook())