libjpeg_turbo: fix race in tests

Fixes:

12/151 Test  #46: jpegtran-shared-icc ...............................***Failed    0.03 sec
Premature end of JPEG file
JPEG datastream contains no image

63/151 Test  #47: jpegtran-shared-icc-cmp ...........................***Failed    0.12 sec
Could not obtain MD5 sum: No such file or directory

The following tests FAILED:
         46 - jpegtran-shared-icc (Failed)
         47 - jpegtran-shared-icc-cmp (Failed)

Example build: https://hydra.nixos.org/build/117594627