chore: improve tests for multipart messages #200

Job	Run time
Dependency review	11s
Test with Codecov coverage (ubuntu-latest / 1.23)	56s
golangci-lint (1.23)	24s
Go vulnerabilities check	17s
REUSE Compliance Check	13s
Test on FreeBSD 13.4'	2m 24s
Test on FreeBSD 14.1	2m 24s
Test on FreeBSD 14.2	2m 20s
Test with SonarQube review (ubuntu-latest / 1.23)	1m 17s
Test (macos-latest / 1.19)	32s
Test (macos-latest / 1.20)	31s
Test (macos-latest / 1.21)	23s
Test (macos-latest / 1.22)	24s
Test (ubuntu-latest / 1.19)	30s
Test (macos-latest / 1.23)	24s
Test (ubuntu-latest / 1.20)	28s
Test (ubuntu-latest / 1.21)	28s
Test (ubuntu-latest / 1.22)	25s
Test (ubuntu-latest / 1.23)	30s
Test (windows-latest / 1.19)	1m 33s
Test (windows-latest / 1.20)	2m 17s
Test (windows-latest / 1.21)	2m 5s
Test (windows-latest / 1.22)	2m 22s
Test (windows-latest / 1.23)	2m 20s
	25m 38s

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