exporter: add tests for a directory with an idmap error #5160

Job	Run time
govulncheck	59s
prepare	5s
binaries (darwin/amd64)	1m 9s
binaries (darwin/arm64)	1m 4s
binaries (linux/amd64)	2m 0s
binaries (linux/arm/v7)	2m 7s
binaries (linux/arm64)	1m 57s
binaries (linux/s390x)	2m 12s
binaries (linux/ppc64le)	2m 9s
binaries (linux/riscv64)	2m 10s
binaries (windows/amd64)	1m 44s
binaries (windows/arm64)	1m 45s
test / prepare	2m 12s
test / run (containerd, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	8m 16s
test / run (containerd-rootless, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integr...	8m 56s
test / run (containerd-1.6, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	7m 39s
test / run (containerd-snapshotter-stargz, ./client ./cmd/buildctl ./worker/containerd ./solver ./fronte...	8m 18s
test / run (oci, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	5m 38s
test / run (oci-rootless, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	5m 10s
test / run (oci-rootless-slirp4netns-detachnetns, ./client ./cmd/buildctl ./worker/containerd ./solver ....	5m 25s
test / run (oci-snapshotter-stargz, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, int...	5m 30s
test / run (./..., 1, integration gateway)	4m 23s
test / run (./client, containerd, nydus, integration)	7m 22s
test / run (./client, oci, nydus, integration)	4m 54s
test / run (./..., nydus, 1, integration)	4m 45s
image	8m 54s
image (rootless)	9m 2s
release	6s
scout	0s
	1h 55m 51s

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