tests/test-username-newline.t

tests: make emacs test less strict

It was failing like this for me:

```
@@ -2,7 +2,7 @@
   $ emacs -q -no-site-file -batch -l $TESTDIR/../contrib/hg-test-mode.el \
   >  -f ert-run-tests-batch-and-exit
   Running 1 tests (*) (glob)
-     passed  1/1  hg-test-mode--compilation-mode-support
+     passed  1/1  hg-test-mode--compilation-mode-support (0.045732 sec)

-  Ran 1 tests, 1 results as expected (*) (glob)
+  Ran 1 tests, 1 results as expected, 0 unexpected (2020-09-11 16:49:56+0000, 0.046034 sec)
```

Differential Revision: https://phab.mercurial-scm.org/D9017

  $ 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]