Add an endline character to InstallerTest::__str__

This is in order for the first ERROR message to come on the next line as opposed to at the end of the long test string, e.g.: http://build.chromium.org/p/chromium.fyi/builders/Chromium%20Win%20MiniInstaller%20Tests/builds/210/steps/test_mini_installer/logs/stdio

NOTRY=True
BUG=264859

Review URL: https://codereview.chromium.org/76433002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236019 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236019