Fix a number of sendable warnings in test utilities #126

Job	Run time
Cxx interop / Construct Cxx interop matrix	2s
Unit tests / Construct unit test matrix	2s
Soundness / Unacceptable language check	4s
Soundness / API breakage check	12s
Soundness / Broken symlinks check	3s
Soundness / Documentation check	12s
Soundness / Format check	10s
Soundness / License headers check	3s
Soundness / Shell check	11s
Soundness / Python lint check	12s
Soundness / YAML lint check	4s
Static SDK / Static SDK / Linux (latest-release Jammy)	23s
Static SDK / Static SDK / Linux (main Jammy)	23s
Cxx interop / Cxx interop / Linux (5.10)	24s
Cxx interop / Cxx interop / Linux (6.0)	25s
Cxx interop / Cxx interop / Linux (6.1)	24s
Cxx interop / Cxx interop / Linux (nightly-next)	24s
Cxx interop / Cxx interop / Linux (nightly-main)	25s
Unit tests / Unit tests / Linux (5.10)	26s
Unit tests / Unit tests / Linux (6.0)	26s
Unit tests / Unit tests / Linux (6.1)	28s
Unit tests / Unit tests / Linux (nightly-next)	26s
Unit tests / Unit tests / Linux (nightly-main)	25s
	6m 14s

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