tests/test-debugbackupbundle.t

run-tests: fix `HGTESTEXTRAEXTENSIONS` with py3

Since `extensions` was a str and `section` bytes, it never populated anything.
If it had, it would have put bytes into the environment dictionary that is all
str.  As everything starts and ends as str, remove the incomplete attempt at
byteification.  It doesn't appear that we had any test coverage of this bit of
code, so also add a non-extension config to make sure it is filtered out
properly.

Differential Revision: https://phab.mercurial-scm.org/D9557

  $ cat >> $HGRCPATH << EOF
  > [extensions]
  > strip=
  > EOF

Setup repo

  $ hg init repo
  $ cd repo

Test backups list and recover

  $ hg debugbackupbundle
  no backup changesets found

  $ mkcommit() {
  >    echo "$1" > "$1"
  >    hg add "$1"
  >    hg ci -l $1
  > }
  $ mkcommit a
  $ mkcommit b
  $ hg strip .
  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
  saved backup bundle to $TESTTMP/repo/.hg/strip-backup/d2ae7f538514-2953539b-backup.hg
  $ hg debugbackupbundle
  Recover changesets using: hg debugbackupbundle --recover <changeset hash>
  
  Available backup changesets:
  * (glob)
  d2ae7f538514 b

  $ hg debugbackupbundle --recover d2ae7f538514
  Unbundling d2ae7f538514
  adding changesets
  adding manifests
  adding file changes
  added 1 changesets with 1 changes to 1 files
  new changesets d2ae7f538514 (1 drafts)