device/raspistill/raspistill_test.go: Raspistill.read() not waiting for more bytes before copying even when not EOF

[Saxon Milton]

Raspistill.read() should block until there is more data to read when it’s not EOF, but this does not happen. We need to check the length of the buffer continually until there’s data. This would obviously need to terminate in the case of device shutdown i.e. resulting in EOF.

[Saxon Milton]

Resolved by PR #452.