test that the tmpdir Miri tests see is absolute and a directory

tests/pass/shims/fs.rs

@@ -15,6 +15,7 @@ use std::io::{Error, ErrorKind, IsTerminal, Read, Result, Seek, SeekFrom, Write}
 use std::path::{Path, PathBuf};
 
 fn main() {
+    test_path_conversion();
     test_file();
     test_file_clone();
     test_file_create_new();
@@ -74,6 +75,12 @@ fn prepare_with_content(filename: &str, content: &[u8]) -> PathBuf {
     path
 }
 
+fn test_path_conversion() {
+    let tmp = tmp();
+    assert!(tmp.is_absolute());
+    assert!(tmp.is_dir());
+}
+
 fn test_file() {
     let bytes = b"Hello, World!\n";
     let path = prepare("miri_test_fs_file.txt");