Tests

Another small simplification of `prepare_test_runner` (#667) #1445

Job	Run time
Run the in-tree version of cargo-insta on ourselves	57s
Test latest deps	1m 54s
Check on MSRV	41s
Test on Latest (ubuntu-latest)	1m 8s
Build docs	29s
Test on Latest (macos-latest)	56s
Test with minimal version resolution	28s
clippy	11s
Test on Latest (windows-latest)	2m 18s
rustfmt	11s
	9m 13s

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