Maint: do not mock os functions

- Use filepaths with /tmp for tests instead
- Only mock functions where filepath can't be specified such as user.Lookup

internal/storage/flush.go

@@ -0,0 +1,7 @@
+package storage
+
+import "github.com/qdm12/gluetun/internal/models"
+
+func (s *storage) FlushToFile(allServers models.AllServers) error {
+	return flushToFile(s.filepath, allServers)
+}