Auto merge of #1506 - RalfJung:test-isolation, r=RalfJung

make sure opening a file fails with isolation enabled

tests/compile-fail/fs/isolated_file.rs

@@ -0,0 +1,6 @@
+// ignore-windows: File handling is not implemented yet
+// error-pattern: `open` not available when isolation is enabled
+
+fn main() {
+    let _file = std::fs::File::open("file.txt").unwrap();
+}