Unit test for reading samples in the padding area.

OpenZGY documents that while you are allowed to read data outside the survey, up to the next brick boundary, the values of those padding samples is unspecified.

However, according to the principle of least surprise, the result should not depend on whether the single-brick shortcut was applied. This pull request adds tests for that. If the strategy changes to always set the padding samples to the default (usually zero) value then the tests can easily be changed to reflect that.