[PROPOSAL] Add more tests to the library (#865)

* phpunit: use dedicate `Tests` namespace for tests

* tests: replace phpunit with orchestra/testbench

* phpunit: ignore result cache file

* phpunit: adapt test for newer version

* tests: add base testcase class with a helper to run/assert commands

* tests: add test for ide-helper:eloquent

* travis: add global default variables

* travis: disable xdebug, speeds up running tests

* travis: convert php versions to build matrix for environment vars

* travis: only run phpcs/phpunit when the env vars signal it

However phpcs only runs with one test combination from the matrix,
not necessary to execute it every time.

.gitignore

@@ -2,3 +2,4 @@ build
 vendor
 composer.lock
 .idea
+.phpunit.result.cache