Updated messagepack_failure and raw_roundtrip tests to expect structured JSON response format instead of raw output, update echo.trampolined

Author: mssalemi

tests/fixtures/build/echo.trampolined.wasm

@@ -393,7 +393,7 @@ mod tests {
         .expect("Failed waiting for output");
 
         cmd.assert().success();
-        cmd.assert().stdout(contains("null"));
+        cmd.assert().stdout(contains("\"output\": {}"));
 
         Ok(())
     }
@@ -419,7 +419,7 @@ mod tests {
 
         child.wait_with_output()?;
         cmd.assert().success();
-        cmd.assert().stdout(contains("7b 7d"));
+        cmd.assert().stdout(contains("\"output\": {}"));
 
         Ok(())
     }