Upgrade our test coverage

[fthouraud]

Our code is overall well covered but some fallbacks could be tested.

Here is a recent report:

--------------------|---------|----------|---------|---------|-------------------
File                | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s 
--------------------|---------|----------|---------|---------|-------------------
All files           |   97.84 |    89.06 |     100 |     100 |                   
 index.js           |    96.8 |    91.67 |     100 |     100 | 81,87,193,198     
 output_adapters.js |     100 |       70 |     100 |     100 | 50,55,57          
 output_utils.js    |     100 |      100 |     100 |     100 |                   
--------------------|---------|----------|---------|---------|-------------------