tests/test-username-newline.t

run-tests: move env into Test

Environment variables are an implementation detail of how tests are
executed. This patch moves environment variable logic into Test and
completely hides it from the outside.

With this patch, a Test can be executed with two lines: init + run().
Tests are still single-use and take a more arguments to the constructor
than likely necessary. These will get addressed in subsequent patches.

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