make TestService_UserReplies more robust

[paskal]

It flaps (see #380), and with this change, it will have more time to get the expected output in the flaky GitHub Actions environment.

The latest example of false positive: link.

[coveralls]

Pull Request Test Coverage Report for Build 1200637388

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage remained the same at 83.702%

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Files with Coverage Reduction	New Missed Lines	%
backend/app/notify/notify.go	2	94.34%

Totals
Change from base Build 1191515627:	0.0%
Covered Lines:	5752
Relevant Lines:	6872

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💛 - Coveralls