Use built binary when executing spectron tests

[pyphilia]

No description provided.

[pyphilia]

I investigated this issue, and editing electronPath is not working for me (Windows with an unpacked build .exe). The built version of graasp-desktop opens but is completely blank. It is not the case when I open the .exe as a normal executable file.

It might work for other OS. The process would be:

• change path: electronPath in the test/application.js file with the executable file (produced with yarn dist)
• change the mocha script in package.json to just
  "mocha": "mkdirp test/tmp && mocha --require @babel/register \"test/**/*.test.js\""
  Here I believe that, if we are using an executable file, the local server should not be necessary.
  You might want to specify a file to just run a single test (and not 50 long tests). test/menu.test.js for instance contains a single test.
• run yarn mocha