Adding tests

Signed-off-by: dubo-dubon-duponey <dubodubonduponey+github@pm.me>

examples/tests/fetch-container/nonexistent/digest/main.cue

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+package testing
+
+#dagger: compute: [
+	{
+		do: "fetch-container"
+		ref: "alpine@sha256:c6c7524e2111f22a9f7577211232d89a9e68cf5b9ed4a41ba77957c9771380a5"
+	}
+]

examples/tests/fetch-container/nonexistent/image-with-valid-digest/main.cue

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+package testing
+
+// XXX WATCHOUT
+// Once buildkit has pulled that digest, it will stay cached and happily succeed WHATEVER the image name then is
+#dagger: compute: [
+	{
+		do: "fetch-container"
+		ref: "busyboxaaa@sha256:e2af53705b841ace3ab3a44998663d4251d33ee8a9acaf71b66df4ae01c3bbe7"
+	},
+]

examples/tests/fetch-container/nonexistent/image/main.cue

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+package testing
+
+#dagger: compute: [
+	{
+		do: "fetch-container"
+		ref: "doesnotexist"
+	}
+]

examples/tests/fetch-container/nonexistent/tag/main.cue

@@ -0,0 +1,8 @@
+package testing
+
+#dagger: compute: [
+	{
+		do: "fetch-container"
+		ref: "alpine:doesnotexist"
+	}
+]