chore: improve tests for multipart messages #198

Job	Run time
Test with Codecov coverage (ubuntu-latest / 1.23)	55s
Go vulnerabilities check	16s
golangci-lint (1.23)	27s
Dependency review	11s
REUSE Compliance Check	17s
Test with SonarQube review (ubuntu-latest / 1.23)	1m 28s
Test on FreeBSD 13.4'	2m 19s
Test on FreeBSD 14.1	2m 22s
Test on FreeBSD 14.2	2m 22s
Test (macos-latest / 1.19)	53s
Test (macos-latest / 1.20)	47s
Test (macos-latest / 1.21)	25s
Test (macos-latest / 1.22)	21s
Test (macos-latest / 1.23)	28s
Test (ubuntu-latest / 1.19)	29s
Test (ubuntu-latest / 1.20)	32s
Test (ubuntu-latest / 1.21)	23s
Test (ubuntu-latest / 1.22)	31s
Test (ubuntu-latest / 1.23)	26s
Test (windows-latest / 1.19)	1m 2s
Test (windows-latest / 1.20)	2m 34s
Test (windows-latest / 1.21)	3m 38s
Test (windows-latest / 1.22)	4m 15s
Test (windows-latest / 1.23)	3m 28s
	30m 49s

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