add ReadAt/Seek test

[btc]

This PR demonstrates a deviation from the io.ReaderAt API contract in the mem.File implementation.

If ReadAt is reading from an input source with a seek offset, ReadAt should not affect nor be affected by the underlying seek offset.

afero/mem/file.go

Line 195 in d40851c

func (f *File) ReadAt(b []byte, off int64) (n int, err error) {

The PR modifies the mem.File impl to respect the contract by storing the original offset before the Read and restoring the offset after the Read.

[CLAassistant]

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