Testsuite: added support for changing and testing an extension's configuration.

Makefile.am

src/lib-sieve/plugins/enotify/ext-enotify.c

src/lib-sieve/plugins/imap4flags/ext-imapflags.c

src/lib-sieve/plugins/include/ext-include.c

...ib-sieve/plugins/spamvirustest/ext-spamvirustest-common.c

src/lib-sieve/plugins/subaddress/ext-subaddress.c

src/lib-sieve/sieve-extensions.c

src/lib-sieve/sieve-extensions.h

src/testsuite/Makefile.am

src/testsuite/cmd-test-config.c

src/testsuite/ext-testsuite.c

src/testsuite/testsuite-common.h

tests/extensions/subaddress/config.svtest