Testsuite: added multiscript tests and required support.

Makefile.am

src/lib-sieve/ext-reject.c

src/lib-sieve/plugins/vacation/cmd-vacation.c

src/testsuite/Makefile.am

src/testsuite/cmd-test-result-print.c

src/testsuite/ext-testsuite.c

src/testsuite/testsuite-common.h

src/testsuite/testsuite-log.c

src/testsuite/testsuite-result.c

src/testsuite/testsuite-result.h

src/testsuite/tst-test-result-execute.c

src/testsuite/tst-test-script-compile.c

src/testsuite/tst-test-script-run.c

tests/multiscript/basic.svtest

tests/multiscript/conflicts.svtest

tests/multiscript/fileinto-inbox.sieve

tests/multiscript/notify.sieve

tests/multiscript/reject-1.sieve

tests/multiscript/reject-2.sieve

tests/multiscript/vacation.sieve