Update expected error messages in tests.

tests/compile-fail/fs/isolated_file.rs

@@ -1,5 +1,5 @@
 // ignore-windows: File handling is not implemented yet
-// error-pattern: `open` not available when isolation is enabled
+// error-pattern: open not available when isolation is enabled
 
 fn main() {
     let _file = std::fs::File::open("file.txt").unwrap();

tests/compile-fail/fs/isolated_stdin.rs

@@ -7,7 +7,7 @@ extern crate libc;
 fn main() -> std::io::Result<()> {
     let mut bytes = [0u8; 512];
     unsafe {
-        libc::read(0, bytes.as_mut_ptr() as *mut libc::c_void, 512); //~ ERROR `read` not available when isolation is enabled
+        libc::read(0, bytes.as_mut_ptr() as *mut libc::c_void, 512); //~ ERROR read not available when isolation is enabled
     }
     Ok(())
 }