Replace numbers with proper HTTP status codes in tests

[paskal]

Also, remove unneeded whitespaces using whitespace linter for golangci-lint.

[coveralls]

Pull Request Test Coverage Report for Build 2181168709

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 10 unchanged lines in 9 files lost coverage.
• Overall coverage decreased (-0.2%) to 84.328%

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Files with Coverage Reduction	New Missed Lines	%
backend/app/migrator/native.go	1	73.17%
backend/app/migrator/wordpress.go	1	80.46%
backend/app/notify/slack.go	1	91.53%
backend/app/rest/api/rest.go	1	93.51%
backend/app/rest/api/rss.go	1	83.48%
backend/app/store/engine/bolt.go	1	83.47%
backend/app/store/image/fs_store.go	1	83.18%
backend/app/store/service/service.go	1	88.27%
backend/app/notify/notify.go	2	94.34%

Totals
Change from base Build 2175145195:	-0.2%
Covered Lines:	6005
Relevant Lines:	7121

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💛 - Coveralls