Add option to speed up the tests. Fix #400

[NicolasCARPi]

• Check if first argument of test.sh is a file instead of its existence
• Add "fast" argument to skip test in "@group slow"
• Small fixes to the bash scripts (quotes, syntax, failsafe)
• Add CONTRIBUTING.md file with instructions for tests and syntax fixing

On my 4 GHz CPU, this brings the tests from 109 seconds down to 63 seconds.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 89.12%. Comparing base (62a8c16) to head (fcfa409). Report is 72 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #442   +/-   ##
=========================================
  Coverage     89.12%   89.12%           
  Complexity      206      206           
=========================================
  Files             9        9           
  Lines           929      929           
=========================================
  Hits            828      828           
  Misses          101      101           

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[NicolasCARPi]

Hello,

I pushed some commits to address your comments and fix the conflict.