tests: fix up test-bad-extension.t's inline extension
Upcoming patches will be simpler, I think.
Differential Revision: https://phab.mercurial-scm.org/D5003
## Avoid interference from actual test env:unset HGTEST_JOBSunset HGTEST_TIMEOUTunset HGTEST_PORTunset HGTEST_SHELL