Correct directory permission of testDir

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.