tests: fix build failure on Go 1.15 and older
tests: fix build failure on Go 1.15 and older

These don't have os.ReadDir yet.

Error was:

	Error: vet: tests/defaults/overlayfs_test.go:104:15: ReadDir not declared by package os

rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs