Testsuite: added support for testing mailbox operations.

src/testsuite/Makefile.am

src/testsuite/cmd-test-mailbox.c

src/testsuite/ext-testsuite.c

src/testsuite/testsuite-common.h

src/testsuite/testsuite-mailstore.c

src/testsuite/testsuite-mailstore.h

src/testsuite/testsuite-message.c

src/testsuite/testsuite-message.h

src/testsuite/testsuite.c

tests/execute/actions.svtest

tests/extensions/imap4flags/execute.svtest