patch: fix import-time syntax error in test-check-module-imports.t
This is more correct, though we were getting lucky in practice with
our rewriter saving us.
# skip-blame just a b prefix
Differential Revision: https://phab.mercurial-scm.org/D3584
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL