coverage: reserve coverage across all multiple running

[xhd2015]

According to https://github.com/golang/go/blob/b68f8ca89a2fb6b33d1e78fadc33d1c35693f6b8/src/cmd/go/internal/test/cover.go#L35

func initCoverProfile() {
	// ...
	f, err := os.Create(testCoverProfile)
	// ...
}

Upon every running, the existing coverage profile will be truncated, so test of previous running does not persist.

We may need to create a new profile on every running and merge all profiles together.