test/send-email: --[no-]xmailer tests
test/send-email: --[no-]xmailer tests

Add tests for the --[no-]xmailer option.

Signed-off-by: Luis Henriques <f59c32834deac78b0d5410e9f6b5d136d75e9fcc@camandro.org>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git