tests: move llb tests into tests/llb

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>

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

@@ -0,0 +1,10 @@
+package testing
+
+// XXX WATCHOUT
+// Once buildkit has pulled that digest, it will stay cached and happily succeed WHATEVER the image name then is
+#compute: [
+	{
+		do:  "fetch-container"
+		ref: "busyboxaaa@sha256:e2af53705b841ace3ab3a44998663d4251d33ee8a9acaf71b66df4ae01c3bbe7"
+	},
+]