Fix test warnings

[TylerPachal]

👋 Hello,

I am getting started with the etso project and before I contribute other changes I wanted to merge this small PR to fix the compiler warnings.

==> etso
Compiling 24 files (.ex)
Generated etso app
warning: variable "a" is unused (if the variable is not meant to be used, prefix it with an underscore)
  test/northwind/repo_test.exs:185: Northwind.RepoTest."test Delete Where"/1

warning: variable "b" is unused (if the variable is not meant to be used, prefix it with an underscore)
  test/northwind/repo_test.exs:185: Northwind.RepoTest."test Delete Where"/1

warning: variable "a" is unused (if the variable is not meant to be used, prefix it with an underscore)
  test/northwind/repo_test.exs:193: Northwind.RepoTest."test Delete Where Select"/1

warning: variable "b" is unused (if the variable is not meant to be used, prefix it with an underscore)
  test/northwind/repo_test.exs:193: Northwind.RepoTest."test Delete Where Select"/1