Add backpack test #3127

Job	Run time
nix-build	1m 6s
unit-tests	5s
cabal-doExactConfig	9s
cabal	9s
runghc	8s
tests-benchmarks	10s
build-extra-hackages	10s
docs	34s
hix	27s
maintainer-scripts	4s
multi-target	28s
plan-extra-hackages	9s
shellFor-hoogle	10s
shellFor-multiple-package	13s
shellFor-not-depends	16s
shellFor-single-package	17s
check-materialization-concurrency	19s
check-path-support	5s
closure-size	2s
haskell-nix-roots-do-not-require-IFDs	8s
hix-cabal	8s
hydra-ifdLevel-0-and-1	1m 51s
hydra-ifdLevel-2	2m 32s
hydra-ifdLevel-3	30m 11s
hydra-without-remote-builders-ghc9102	50s
hydra-without-remote-builders-ghc967	59s
nix-tools	11s
update-docs	4s
	41m 55s

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