header: increase test coverage for Store

[Wondertan]

[rootulp]

Perhaps useful for assignee, you can generate code coverage via:

cd service/header
go test . --coverprofile=coverage.out
go tool cover -html=coverage.out

service/header/store.go is at 79.9% test coverage. The uncovered lines are primarily error handling lines. It looks like https://github.com/celestiaorg/celestia-node/blob/main/service/header/store_test.go doesn't test for error handling yet.

[HoytRen]

I guess this is out of date since here isn't service/header now.

[rootulp]

@HoytRen it looks like it was moved to https://github.com/celestiaorg/celestia-node/tree/main/header

For example: https://github.com/celestiaorg/celestia-node/blob/main/header/store/store.go