pkg/covermerger: more tests #14936

Job	Run time
aux	4m 55s
arch (presubmit_arch_linux)	10m 48s
arch (presubmit_arch_freebsd)	6m 29s
arch (presubmit_arch_netbsd)	4m 27s
build	10m 44s
arch (presubmit_arch_openbsd)	4m 20s
arch (presubmit_arch_darwin)	4m 23s
dashboard	3m 42s
gvisor	4m 12s
arch (presubmit_arch_windows)	3m 18s
old	6m 44s
arch (presubmit_arch_executor)	3m 16s
race	8m 55s
race_dashboard	4m 52s
	1h 21m 5s

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