Fix test-only failure for ghcjs #2010

Job	Run time
cabal	9s
cabal-doExactConfig	14s
nix-build	50s
unit-tests	9s
runghc	11s
build-extra-hackages	6s
docs	23s
hix	32s
hydra-ifdLevel-0-and-1	14s
maintainer-scripts	11s
multi-target	16s
plan-extra-hackages	6s
shellFor-hoogle	9s
shellFor-multiple-package	14s
shellFor-not-depends	12s
shellFor-single-package	17s
tests-benchmarks	7s
check-materialization-concurrency	20s
check-path-support	6s
closure-size	4s
haskell-nix-roots-do-not-require-IFDs	7s
hix-cabal	20s
hydra-ifdLevel-2	12s
hydra-ifdLevel-3-and-ghc-8-10	9s
hydra-ifdLevel-3-and-ghc-9-2	13s
hydra-ifdLevel-3-and-not-ghc-8-10-or-9-2	10s
hydra-without-remote-builders	36s
nix-tools	13s
update-docs	8s
	6m 58s

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