Add is_instance test #481

Job	Run time
rust_ci (macos-latest, stable)	58s
rust_ci (macos-latest, beta)	1m 0s
rust_ci (macos-latest, nightly)	55s
rust_ci (ubuntu-latest, stable)	1m 5s
rust_ci (ubuntu-latest, beta)	51s
rust_ci (ubuntu-latest, nightly)	1m 2s
rust_ci (windows-latest, stable)	2m 39s
rust_ci (windows-latest, beta)	1m 30s
rust_ci (windows-latest, nightly)	1m 53s
	11m 53s

Provide feedback