Improve tests

[jmattheis]

• fix a bug where messages were not deleted correctly
• replace mocks (because they suck) with an in memory sqlite database

[codecov-io]

Codecov Report

Merging #36 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #36   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          21     24    +3     
  Lines         538    612   +74     
=====================================
+ Hits          538    612   +74

Impacted Files	Coverage Δ
api/stream/stream.go	100% <ø> (ø)
api/message.go	100% <ø> (ø)	⬆️
auth/password/password.go	100% <ø> (ø)
config/config.go	100% <ø> (ø)	⬆️
api/stream/client.go	100% <ø> (ø)
mode/mode.go	100% <100%> (ø)	⬆️
api/internalutil.go	100% <100%> (ø)	⬆️
database/database.go	100% <100%> (ø)	⬆️
test/asserts.go	100% <100%> (ø)
auth/authentication.go	100% <100%> (ø)	⬆️
... and 8 more

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