[WIP] Add a test to reproduce error #646

Job	Run time
format	17s
test (ubuntu-latest)	42s
check-doc	22s
test (ubuntu-latest, flexible-string)	36s
check (ubuntu-latest)	18s
test (ubuntu-latest, source-location)	27s
check-msrv (ubuntu-latest)	36s
check (ubuntu-latest, x86_64-apple-ios)	27s
test (ubuntu-latest, log native libsystemd multi-thread)	30s
check-msrv (windows-latest)	52s
clippy (ubuntu-latest)	1m 38s
check (windows-latest)	50s
check-msrv (macos-latest)	44s
test (ubuntu-latest, log native libsystemd multi-thread, flexible-string)	29s
clippy (windows-latest)	2m 42s
check (windows-latest, x86_64-apple-ios)	43s
test (ubuntu-latest, log native libsystemd multi-thread, source-location)	28s
clippy (macos-latest)	1m 34s
check (macos-latest)	18s
test (windows-latest)	1m 17s
check (macos-latest, x86_64-apple-ios)	21s
test (windows-latest, flexible-string)	55s
test (windows-latest, source-location)	1m 18s
test (windows-latest, log native libsystemd multi-thread)	1m 16s
test (windows-latest, log native libsystemd multi-thread, flexible-string)	1m 32s
test (windows-latest, log native libsystemd multi-thread, source-location)	1m 7s
test (macos-latest)	32s
test (macos-latest, flexible-string)	42s
test (macos-latest, source-location)	34s
test (macos-latest, log native libsystemd multi-thread)	35s
test (macos-latest, log native libsystemd multi-thread, flexible-string)	39s
test (macos-latest, log native libsystemd multi-thread, source-location)	31s
	25m 52s

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