Support custom binary paths based on env variables

Support custom binary paths based on env variables (#30)

* Fix typo of var hint in SnipeMigrationsTest

* Fix incorrect custom binary example case

* Support custom binary paths based on env variables

* Fix style issues in SnipeCustomBinaryTest

* Decrease unit test scope for .env binary paths

No need to spin up the whole thing just to see if the config reflects our changes

* Don’t run tests twice in custom binary test class

Moving the helper functions to the base test class we can now extend that one intead. Otherwise, all tests would have been executed twice.

* Add custom binary path feature to CHANGELOG.md