Add --json to log_cat to print fully valid json

This makes it easy to use JSON based tooling to dig inside the results.

Change-Id: I483869fe27e117bdf86b9f775371842477ef1cbe
Signed-off-by: Austin Schuh <austin.linux@gmail.com>
diff --git a/aos/flatbuffer_introspection_test.cc b/aos/flatbuffer_introspection_test.cc
index fe8460f..e435709 100644
--- a/aos/flatbuffer_introspection_test.cc
+++ b/aos/flatbuffer_introspection_test.cc
@@ -362,7 +362,7 @@
   std::string out =
       FlatbufferToJson(schema_, builder.GetBufferPointer(),
                        {.multi_line = false, .max_vector_size = 100});
-  EXPECT_EQ(out, "{\"vector_foo_int\": [ ... 101 elements ... ]}");
+  EXPECT_EQ(out, "{\"vector_foo_int\": [ \"... 101 elements ...\" ]}");
 }
 
 TEST_F(FlatbufferIntrospectionTest, MultilineTest) {