Fixes compilation warning from Instance.handle_call/3

[antedeguemon]

Hey! 🖖

This pull request:

• Groups Instance.handle_call/3 together to fix the warning
• Adds a new CI step to verify for compilation warnings

I caught the warning log when compiling a project that depends on test_server:

==> test_server
Compiling 12 files (.ex)
     warning: clauses with the same name and arity (number of arguments) should be grouped together, "def handle_call/3" was previously defined (lib/test_server/instance.ex:184)
     │
 246 │   def handle_call(
     │       ~
     │
     └─ lib/test_server/instance.ex:246:7

[danschultzer]

Thanks ❤️