tests/test-username-newline.t

run-tests: make code coverage work on Python 3

This code path was obviously not tested on Python 3 because it
blew up in several places due to str/bytes mismatch.

For internal code, we normalize paths to bytes.

For code calling into `coverage`, we normalize paths to str,
which is what `coverage` seems to expect.

After this, `run-tests.py -H` works on Python 3!

Differential Revision: https://phab.mercurial-scm.org/D7133

  $ 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]