While running unit tests for perf on a Kube cluster with a memory backed
emptydir as TMPDIR, TestSafeMakeDir failed with:

```
--- FAIL: TestSafeMakeDir (0.01s)
	mount_linux_test.go:661: test "directory-exists": expected permissions 20000000750, got 20020000750
```

(TMPDIR set to /tmp/volume, /tmp/volume is EmptyDir with type Memory)

The test doesn't actually care about `os.ModeSetgid`, so specifically mask it out when testing this way.