tests/test-username-newline.t

tests: make test-install.t work on debian systems

Debian systems, at least as of their version of python3.8 on my machine, have
rewritten some logic in ensurepip to make it not use the wheels in pip._bundled,
but instead to use wheels installed in /usr/share/python-wheels. It copies these
wheels into the virtual environment when it's created, and installenv/bin/pip is
able to see them and use them, so it thinks that 'wheel' is installed, and that
it can build the mercurial wheel instead of just installing it. For some reason,
when it subprocesses to run `python3 setup.py bdist_wheel`, it setup.py does
*not* have the 'wheel' wheel available, and we get an error message.

Differential Revision: https://phab.mercurial-scm.org/D8813

  $ hg init
  $ touch a

  $ unset HGUSER
  $ echo "[ui]" >> .hg/hgrc
  $ echo "username= foo" >> .hg/hgrc
  $ echo "          bar1" >> .hg/hgrc

  $ hg ci -Am m
  adding a
  abort: username 'foo\nbar1' contains a newline
  
  [255]
  $ rm .hg/hgrc

  $ HGUSER=`(echo foo; echo bar2)` hg ci -Am m
  adding a
  abort: username 'foo\nbar2' contains a newline
  
  [255]
  $ hg ci -Am m -u "`(echo foo; echo bar3)`"
  adding a
  transaction abort!
  rollback completed
  abort: username 'foo\nbar3' contains a newline!
  [255]