[PROPOSAL] Add more tests to the library

[mfn]

I don't know how others feel, but many of my projects depend on this library.

Eventually I would like to contribute, give back something to this great idea of the author and the community. My PR #859 was merged and I really wanted to write a test, but I realized: there is nothing (ok, 2 tests…) but also no infrastructure for running these kind of tests (Laravel framework setup, models to test, etc;) 😢

Laravel packages whose functionality depends on the Laravel framework cannot easily be tested just with PHPUnit alone and usually require https://github.com/orchestral/testbench

So what I did in this PR

• (chore: moved tests into a dedicated Tests namespace)
• removed phpunit and added orchestral/testbench (which itselds depends on PHPUnit…)
• picked one small piece of code and wrote a test for it, to get things kickstarted
• reworked travis and
  • removed actual running unit tests on very old PHP/Laravel versions (see below)
  • only run phpcs in one combination (usually the latest)

Immediate issues I encountered

• According to the composer.json version constraints, this library supports VERY OLD versions of Laravel
• composer could not resolve any combination for orchestral/testbanch for the older Laravel versions, I got multi-page of package resolution errors
• I could not find a sane way to keep the old tests, depending on PHPUnit_Framework_TestCase, working with the supported versions of orchestral because it depends on newer phpunit version now aliasing PHPUnit_Framework_TestCase to PHPUnit\Framework\Testcase anymore

Since I could not find a solution to this, I simply decided to:

• remove running the tests for these old versions/combinations
• but keep the old versions in travis because they still run composer install and ensure the combination itself works

What does this mean?

• I think eventually at some point the version constraints should be cleaned up and non supported versions probably be dropped. But I don't want to make this call and not in this PR
• I hope to make it easier for others to not just contribute features but also add tests for them

I certainly offer to add more tests to bring them up to speed, but before investing more time I would first like this approach to be approved, I hope this is understandable 😄

[barryvdh]

I think we can bump the minimum version to laravel 5.5 (and the php version that supports that), or higher if that makes it easier.

[barryvdh]

And nice work :)
Should I merge this or should we bump the versions and simplify it a bit?

[mfn]

Merge is fine, I'll follow up with your suggestions!