Tests

Small simplification of `prepare_test_runner` #1441

Job	Run time
Check on MSRV	42s
Run the in-tree version of cargo-insta on ourselves	50s
Test on Latest (ubuntu-latest)	1m 13s
Test on Latest (macos-latest)	1m 2s
Build docs	24s
Test latest deps	1m 46s
Test on Latest (windows-latest)	3m 27s
Test with minimal version resolution	22s
clippy	13s
rustfmt	6s
	10m 5s

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