Testsuite: added storage of outgoing SMTP messages.

src/testsuite/Makefile.am

src/testsuite/testsuite-common.c

src/testsuite/testsuite-common.h

src/testsuite/testsuite-message.c

src/testsuite/testsuite-smtp.c

src/testsuite/testsuite-smtp.h

src/testsuite/testsuite.c

src/testsuite/tst-test-script-run.c