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/json_to_flatbuffer_test.cc b/aos/json_to_flatbuffer_test.cc
index e353b99..42457f6 100644
--- a/aos/json_to_flatbuffer_test.cc
+++ b/aos/json_to_flatbuffer_test.cc
@@ -270,9 +270,9 @@
 
   EXPECT_EQ(json_short, back_json_short_typetable);
   EXPECT_EQ(json_short, back_json_short_reflection);
-  EXPECT_EQ("{ \"vector_foo_int\": [ ... 101 elements ... ] }",
+  EXPECT_EQ("{ \"vector_foo_int\": [ \"... 101 elements ...\" ] }",
             back_json_long_typetable);
-  EXPECT_EQ("{ \"vector_foo_int\": [ ... 101 elements ... ] }",
+  EXPECT_EQ("{ \"vector_foo_int\": [ \"... 101 elements ...\" ] }",
             back_json_long_reflection);
 }