Use dummy password hasher in tests

[korycins]

This changes will reduce the time of tests execution (#2120).

Config	Master	Branch
-n=2 --reuse-db;, docker 1cpu, 1gb ram	302.81s	167.61s
-n=4 --reuse-db;, docker 1cpu, 2gb ram	250.28s	145.44s

I know that the above results can be not very precise but I hope that all of us will see the differences.

1. Privileged views and APIs are guarded by proper permission checks.
2. All visible strings are translated with proper context.
3. All data-formatting is locale-aware (dates, numbers, and so on).
4. Database queries are optimized and the number of queries is constant.
5. Database migration files are up to date.
6. The changes are tested.
7. The code is documented (docstrings, project documentation).
8. GraphQL schema and type definitions are up to date.
9. Changes are mentioned in the changelog.

[korycins]

FYI @akjanik @NyanKiyoshi

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@8073d95). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #4083   +/-   ##
=========================================
  Coverage          ?   91.56%           
=========================================
  Files             ?      277           
  Lines             ?    15106           
  Branches          ?     1474           
=========================================
  Hits              ?    13832           
  Misses            ?      877           
  Partials          ?      397

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[NyanKiyoshi]

Those are undefined (_).

[NyanKiyoshi]

I fixed it in #3852 (it was failing the checks, thus blocking me from committing).

[korycins]

@NyanKiyoshi good catch. Thanks!