Enable job discard, exporter stub, context cancel bug #195

Job	Run time
prepare	1s
test / prepare	1m 46s
test / run (containerd, ./frontend/dockerfile, integration)	3m 48s
test / run (containerd, ./frontend/dockerfile, dockerfile)	4m 12s
test / run (containerd-rootless, ./frontend/dockerfile, integration)	3m 40s
test / run (containerd-rootless, ./frontend/dockerfile, dockerfile)	4m 15s
test / run (containerd-1.6, ./frontend/dockerfile, integration)	4m 5s
test / run (containerd-1.6, ./frontend/dockerfile, dockerfile)	4m 16s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, integration)	3m 14s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	4m 29s
test / run (oci, ./frontend/dockerfile, integration)	4m 18s
test / run (oci, ./frontend/dockerfile, dockerfile)	3m 49s
test / run (oci-rootless, ./frontend/dockerfile, integration)	4m 16s
test / run (oci-rootless, ./frontend/dockerfile, dockerfile)	3m 46s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, integration)	3m 16s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	3m 50s
image	0s
release	0s
	57m 1s

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