Testsuite: implemented testsuite object interface.

src/lib-sieve/sieve-code.h

src/lib-sieve/sieve-extensions-private.h

src/testsuite/Makefile.am

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