This PR simplifies tests by removing redundant `os.Exit` calls in
`TestMain` functions.
As of Go 1.15, we do not need to call `os.Exit(m.Run())` explicitly, as
value returned by `m.Run()` is stored into unexported field of `m` and
go test executable is smart enough to automatically call
`os.Exit(retValue)` when `TestMain` returns. See golang/go#34129.
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