Testsuite: added support for test object members.

src/testsuite/cmd-test-set.c

src/testsuite/ext-testsuite.c

src/testsuite/tests/testsuite.sieve

src/testsuite/testsuite-common.c

src/testsuite/testsuite-common.h

src/testsuite/testsuite-objects.c

src/testsuite/testsuite-objects.h