Testsuite: added test_fail command.

src/lib-sieve/sieve-generator.c

src/lib-sieve/sieve-generator.h

src/testsuite/Makefile.am

src/testsuite/cmd-test-fail.c

src/testsuite/cmd-test.c

src/testsuite/ext-testsuite.c

src/testsuite/tests/testsuite.sieve

src/testsuite/testsuite-common.c

src/testsuite/testsuite-common.h