chore: improve tests for multipart messages #197

Job	Run time
Test with Codecov coverage (ubuntu-latest / 1.23)	58s
Test with Codecov coverage (ubuntu-latest / 1.23)	58s
Dependency review	11s
Dependency review	11s
Go vulnerabilities check	20s
Go vulnerabilities check	20s
golangci-lint (1.23)	27s
golangci-lint (1.23)	27s
Test with SonarQube review (ubuntu-latest / 1.23)	1m 29s
Test with SonarQube review (ubuntu-latest / 1.23)	1m 29s
REUSE Compliance Check	14s
REUSE Compliance Check	14s
Test on FreeBSD 13.4'	54s
Test on FreeBSD 13.4'	2m 21s
Test on FreeBSD 14.0	49s
Test on FreeBSD 14.0	2m 23s
Test on FreeBSD 14.1	53s
Test on FreeBSD 14.1	2m 23s
Test (macos-latest / 1.19)	44s
Test (macos-latest / 1.19)	44s
Test (macos-latest / 1.20)	35s
Test (macos-latest / 1.20)	35s
Test (macos-latest / 1.21)	27s
Test (macos-latest / 1.21)	27s
Test (macos-latest / 1.23)	26s
Test (macos-latest / 1.23)	26s
Test (ubuntu-latest / 1.19)	33s
Test (ubuntu-latest / 1.19)	33s
Test (macos-latest / 1.22)	21s
Test (macos-latest / 1.22)	21s
Test (ubuntu-latest / 1.20)	32s
Test (ubuntu-latest / 1.20)	32s
Test (ubuntu-latest / 1.21)	27s
Test (ubuntu-latest / 1.21)	27s
Test (ubuntu-latest / 1.22)	21s
Test (ubuntu-latest / 1.22)	21s
Test (ubuntu-latest / 1.23)	22s
Test (ubuntu-latest / 1.23)	22s
Test (windows-latest / 1.19)	1m 10s
Test (windows-latest / 1.19)	1m 10s
Test (windows-latest / 1.20)	2m 17s
Test (windows-latest / 1.20)	2m 17s
Test (windows-latest / 1.21)	1m 51s
Test (windows-latest / 1.21)	1m 51s
Test (windows-latest / 1.22)	1m 51s
Test (windows-latest / 1.22)	1m 51s
Test (windows-latest / 1.23)	54s
Test (windows-latest / 1.23)	54s
	42m 43s

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