tests/test-gendoc-sv.t

rust: update minimum supported Rust compiler version

Debian stable backported `rustc 1.41.1` for a Firefox release that needed it,
so we can finally update the compiler version to something that isn't more than
a year old.

This means that (among other things [1]), `async await` syntax is usable!

Differential Revision: https://phab.mercurial-scm.org/D9138

#require docutils gettext

  $ $TESTDIR/check-gendoc sv
  checking for parse errors