This implements glandium's suggestion from
https://bugzilla.mozilla.org/show_bug.cgi?id=1227248#c5: since it's
not easy to run xpcshell tests in --disable-compile-environment builds
(and, right now, in artifact builds), let's just skip this work
entirely in those situations. This saves about 30s of build time on
my machine.
--HG--
extra : commitid : 5IYhHeJlUY3
extra : rebase_source : 3a7b1cf36f45c40f804aad24ba503a5c5863407e
extra : amend_source : 505dcee2221f600962b35a71ee49192b09fdf40c
extra : histedit_source : 96284d832f27f0c734466e63fb6360f49fb6172b
