Add a option to dry run email commands, add tests of those commands

We have not previously tested LNT email operations. They break sometimes. This patch adds a —-dry-run (-n) flag to disable the actual sending of the emails so the test suite can call the email commands without sending emails.

Add a test which calls the two main email commands, send-run-comparison, and send-daily-report.

git-svn-id: https://llvm.org/svn/llvm-project/lnt/trunk@213338 91177308-0d34-0410-b5e6-96231b3b80d8