Switch to a text summary reporter for JavaScript test coverage.

[tristandunn]

The file list is nice when test coverage is missing, but when at 100% it's not very informative.

Before

$ yarn test:coverage


  ........................................................

  56 passing (176ms)

------------------------------------------|---------|----------|---------|---------|-------------------
File                                      | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s
------------------------------------------|---------|----------|---------|---------|-------------------
------------------------------------------|---------|----------|---------|---------|-------------------

After

$ yarn test:coverage


  ........................................................

  56 passing (177ms)


=============================== Coverage summary ===============================
Statements   : 100% ( 1089/1089 )
Branches     : 100% ( 191/191 )
Functions    : 100% ( 24/24 )
Lines        : 100% ( 1089/1089 )
================================================================================