Fix a number of sendable warnings in test utilities #127

Job	Run time
Unit tests / Construct unit test matrix	3s
Cxx interop / Construct Cxx interop matrix	2s
Soundness / API breakage check	4m 24s
Soundness / Documentation check	2m 49s
Soundness / Unacceptable language check	2s
Soundness / License headers check	3s
Soundness / Broken symlinks check	5s
Soundness / Format check	30s
Soundness / Shell check	31s
Soundness / YAML lint check	4s
Soundness / Python lint check	7s
Static SDK / Static SDK / Linux (latest-release Jammy)	4m 8s
Static SDK / Static SDK / Linux (main Jammy)	2m 5s
Cxx interop / Cxx interop / Linux (5.10)	2m 39s
Cxx interop / Cxx interop / Linux (6.0)	2m 49s
Cxx interop / Cxx interop / Linux (6.1)	2m 53s
Cxx interop / Cxx interop / Linux (nightly-next)	3m 38s
Cxx interop / Cxx interop / Linux (nightly-main)	3m 37s
Unit tests / Unit tests / Linux (5.10)	4m 46s
Unit tests / Unit tests / Linux (6.0)	5m 11s
Unit tests / Unit tests / Linux (6.1)	5m 10s
Unit tests / Unit tests / Linux (nightly-next)	5m 53s
Unit tests / Unit tests / Linux (nightly-main)	5m 59s
	57m 28s

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