tests: fix command name in test-blackbox.t to be bytes
This command is only used when tests are run with --with-chg, so this was missed
before.
Differential Revision: https://phab.mercurial-scm.org/D7553
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL