Testsuite: started support for error validation.

src/testsuite/Makefile.am

src/testsuite/cmd-test-fail.c

src/testsuite/cmd-test-set.c

src/testsuite/ext-testsuite.c

src/testsuite/tests/errors.svtest

src/testsuite/tests/header-errors.sieve

src/testsuite/testsuite-common.h

src/testsuite/tst-test-compile.c

src/testsuite/tst-test-error.c