Add backpack test #2686

Job	Run time
unit-tests	5s
maintainer-scripts	7s
cabal	9s
multi-target	28s
cabal-doExactConfig	13s
nix-build	42s
runghc	7s
shellFor-hoogle	7s
shellFor-multiple-package	14s
shellFor-not-depends	16s
shellFor-single-package	14s
tests-benchmarks	8s
build-extra-hackages	7s
check-materialization-concurrency	20s
check-path-support	5s
closure-size	7s
docs	21s
haskell-nix-roots-do-not-require-IFDs	8s
hix	24s
hix-cabal	7s
hydra-ifdLevel-0-and-1	2m 30s
hydra-ifdLevel-2	2m 35s
hydra-ifdLevel-3	19m 2s
hydra-without-remote-builders-ghc8107	36s
hydra-without-remote-builders-ghc9101	33s
nix-tools	13s
plan-extra-hackages	6s
update-docs	4s
	30m 8s

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