tests/test-backwards-remove.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

  $ hg init
  $ echo This is file a1 > a
  $ hg add a
  $ hg commit -m "commit #0"
  $ ls -A
  .hg
  a
  $ echo This is file b1 > b
  $ hg add b
  $ hg commit -m "commit #1"
  $ hg co 0
  0 files updated, 0 files merged, 1 files removed, 0 files unresolved

B should disappear

  $ ls -A
  .hg
  a