pkg/coveragedb: test SaveMergeResult #14879

Job	Run time
aux	4m 53s
arch (presubmit_arch_linux)	10m 56s
arch (presubmit_arch_freebsd)	6m 35s
arch (presubmit_arch_netbsd)	4m 23s
build	10m 48s
arch (presubmit_arch_openbsd)	4m 22s
arch (presubmit_arch_darwin)	4m 27s
arch (presubmit_arch_windows)	3m 23s
dashboard	3m 23s
arch (presubmit_arch_executor)	3m 22s
old	6m 32s
gvisor	4m 18s
race	8m 33s
race_dashboard	5m 22s
	1h 21m 17s

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