Add a test for std.fs.File's `setEndPos` (which is a simple wrapper around
`std.posix.ftruncate`) to exercise some success and failure paths.
Explicitly check that the `ftruncate` length isn't negative when
interpreted as a signed value. This avoids having to decode overloaded
`EINVAL` errors.
Add errno handling to Windows path to map INVALID_PARAMETER to FileTooBig.
Fixes#22960