pkg/covermerger: more tests #14925

Job	Run time
aux	5m 9s
build	3m 19s
dashboard	3m 47s
arch (presubmit_arch_linux)	10m 45s
gvisor	4m 18s
old	6m 32s
arch (presubmit_arch_freebsd)	6m 28s
race	8m 27s
arch (presubmit_arch_netbsd)	4m 22s
race_dashboard	4m 53s
arch (presubmit_arch_openbsd)	4m 27s
arch (presubmit_arch_darwin)	4m 24s
arch (presubmit_arch_windows)	3m 22s
arch (presubmit_arch_executor)	3m 17s
	1h 13m 30s

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