Add test coverage

[ehmicky]

Fixes #64.

Output on Windows (since some logic is only run on Windows):

-------------|---------|----------|---------|---------|-------------------
File         | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s 
-------------|---------|----------|---------|---------|-------------------
All files    |   99.74 |    98.73 |     100 |   99.74 |                   
 context.js  |     100 |      100 |     100 |     100 |                   
 index.js    |     100 |      100 |     100 |     100 |                   
 iterable.js |   98.38 |    96.15 |     100 |   98.38 | 61                
 options.js  |     100 |      100 |     100 |     100 |                   
 pipe.js     |     100 |      100 |     100 |     100 |                   
 result.js   |     100 |    97.22 |     100 |     100 | 33                
 spawn.js    |     100 |      100 |     100 |     100 |                   
 windows.js  |     100 |      100 |     100 |     100 |                   
-------------|---------|----------|---------|---------|-------------------

Uncovered line in iterable.js is in a catch block that does await iterator.throw(error) which always throws (which c8 cannot guess).
Uncovered line in result.js is after a for await (...) loop which is intentionally never ending (which c8 cannot guess).

We could add c8 disable comments, but they seem to disagree with xo, and add more bytes to the npm install size, so I left it as is.