Add test verifying that line endings are maintained for read_file()

[craigwalton-dsit]

There is a new requirement in Inspect's SandboxEnvironment.read_file()

When reading text files, implementations should preserve newline constructs (e.g. crlf should be preserved not converted to lf).

https://github.com/UKGovernmentBEIS/inspect_ai/blob/bcf7bd58d083073e79a078172887e08341211426/src/inspect_ai/util/_sandbox/environment.py#L205C1-L208C1

This was already the case for the K8s sandbox provider. I've just added a test to prevent any regressions now that this is documented.

The read_file() function should should preserve newline constructs (e.g. crlf should be preserved not converted to lf). This is equivalent to specifying newline="" in a call to the Python open() function.