Update README coverage tables to X/Y format #42

Job	Run time
test-linux	3m 25s
test-macos	2m 22s
cross-compile (aarch64-linux-gnu)	3m 22s
cross-compile (aarch64-macos-none)	3m 24s
cross-compile (x86_64-macos-none)	3m 22s
cross-compile (x86_64-linux-gnu)	3m 16s
	19m 11s

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