Add dummy googletest

Uses the built in cmake -Dtest=ON

It should download googletest on the fly if needed.

Adds a new make target, `make test`

Ref #167

cockatrice/test/dummy_test.cpp

@@ -0,0 +1,16 @@
+#include "gtest/gtest.h"
+
+namespace {
+    class FooTest : public ::testing::Test {
+
+    };
+
+    TEST(DummyTest, Works) {
+        ASSERT_EQ(1, 1) << "One is not equal to one";
+    }
+}
+
+int main(int argc, char **argv) {
+    ::testing::InitGoogleTest(&argc, argv);
+    return RUN_ALL_TESTS();
+}