Fix test-only failure for ghcjs (#2180) #2011

Job	Run time
check-materialization-concurrency	24s
build-extra-hackages	12s
cabal	14s
cabal-doExactConfig	13s
check-path-support	7s
closure-size	7s
docs	39s
haskell-nix-roots-do-not-require-IFDs	9s
hix	1m 2s
hix-cabal	2m 40s
hydra-ifdLevel-2	10s
hydra-ifdLevel-0-and-1	18s
hydra-ifdLevel-3-and-ghc-8-10	11s
hydra-ifdLevel-3-and-ghc-9-2	11s
hydra-ifdLevel-3-and-not-ghc-8-10-or-9-2	6s
hydra-without-remote-builders	28s
maintainer-scripts	7s
multi-target	17s
nix-build	37s
nix-tools	54s
plan-extra-hackages	8s
runghc	8s
shellFor-hoogle	14s
shellFor-multiple-package	11s
shellFor-not-depends	14s
shellFor-single-package	18s
tests-benchmarks	8s
unit-tests	9s
update-docs	5s
	10m 41s

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