pkg/covermerger: more tests #14932

Job	Run time
aux	4m 49s
dashboard	3m 38s
arch (presubmit_arch_linux)	11m 14s
build	11m 12s
arch (presubmit_arch_freebsd)	6m 42s
gvisor	4m 14s
arch (presubmit_arch_netbsd)	4m 20s
old	6m 41s
race	8m 29s
race_dashboard	4m 26s
arch (presubmit_arch_openbsd)	4m 19s
arch (presubmit_arch_darwin)	4m 22s
arch (presubmit_arch_windows)	3m 23s
arch (presubmit_arch_executor)	3m 18s
	1h 21m 7s

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