When fs.MkdirAll() was called with a permission of "777" (without the 0 prefix),
it generated the testDir with a permission of `dr----x--x`, causing the following error
during a test run:
=== RUN TestRead0
--- FAIL: TestRead0 (0.00 seconds)
fs_test.go:53: OsFs create failed: open /tmp/fun/test.txt: permission denied
Changing the decimal `777` to octal `0777` fixes the problem.