Check IO errors in test using raw_os_error() instead of kind()

std::io::ErrorKind is a `#[non_exhaustive]` enum as more specific
error types are to be added in the future. It was unclear in the
docs until very recently, however, that this is to be done by
re-defining `ErrorKind::Other` errors to new enum variants. Thus,
our tests which check explicitly for `ErrorKind::Other` as a
result of trying to access a directory as a file were incorrect.
Sadly, these generated no meaningful feedback from rustc at all,
except that they're suddenly failing in rustc beta!

After some back-and-forth, it seems rustc is moving forward
breaking existing code in future versions, so we move to the
"correct" check here, which is to check the raw IO error.

See rust-lang/rust#86442 and rust-lang/rust#85746 for more info.

lightning-persister/src/util.rs

@@ -178,7 +178,7 @@ mod tests {
 		match write_to_file(path, filename, &test_writeable) {
 			Err(e) => {
 				#[cfg(not(target_os = "windows"))]
-				assert_eq!(e.kind(), io::ErrorKind::Other);
+				assert_eq!(e.raw_os_error(), Some(libc::EISDIR));
 				#[cfg(target_os = "windows")]
 				assert_eq!(e.kind(), io::ErrorKind::PermissionDenied);
 			}