[BE] Small details on LoginControllerTest and AdminController

[FranEnLaNube]

@elpupas , these are some contributions to fix #39, maybe it fixes it.

Changes

I just made changes in these files, the rest has been PINT 🤷‍♂️

LoginControllerTest.php

• Added statement use DatabaseTransactions to avoid refresh data base on each testing or using use refreshDatabase, which resets database in every testing, leaving it totally empty.
• Delete some unused declared variables.
• Changed confusing names on some methods
• Changed routes from $response = $this->json('POST', '/api/v1/login', $credentials); to $response = $this->post(route('login'), $credentials); to avoid SonarCloud messages given that there were string repetition.

AdminController.php

• Improve indentation to solve SonarCloud messages and take the variable $admin of from findAdmin()

[SectionOne]

Revisat, aparentment s'han corregit els punts d'optimització i alineació del codi.