tests: added tests to the account domain

[fmartingr]

Several people has been telling me that the Remember Me check is not working, so I have added a few tests to ensure it's working.

So far I haven't seen any issues, if any of you landing here experience some weird behaviour please open an issue so I can investiagate further.

Refactored the code a bit and added a testutil.GetValidAccount() to be used for authenticated tests and prevent discrepancies between manually set up test code (more notably the logged in user requiring an ID).

[codecov]

Codecov Report

Attention: Patch coverage is 91.66667% with 3 lines in your changes missing coverage. Please review.

Project coverage is 34.74%. Comparing base (8c35a6b) to head (7a606dc).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/domains/accounts.go	87.50%	2 Missing ⚠️
internal/http/routes/api/v1/auth.go	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #993      +/-   ##
==========================================
- Coverage   35.33%   34.74%   -0.60%     
==========================================
  Files          61       61              
  Lines        4078     5322    +1244     
==========================================
+ Hits         1441     1849     +408     
- Misses       2414     3249     +835     
- Partials      223      224       +1     

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