test: Set state_dir in Dovecot config

Dovecot's `state_dir` usually defaults to be at `/var/lib/dovecot`, or a
similar system-wide path.

Under some conditions, our test Dovecot instance can fail, because it's
wanting to write to state_dir, but it is not writeable by us in the test
environment.

This was reported by foxcpp in
https://github.com/albertito/chasquid/issues/28.

This patch fixes the problem by setting a custom state_dir to be within
our testing directory.

Thanks to foxcpp for reporting this problem and suggesting a fix.

test/t-11-dovecot/config/dovecot.conf.in

@@ -1,4 +1,5 @@
 base_dir = $ROOT/run/
+state_dir = $ROOT/lib/
 log_path = $ROOT/dovecot.log
 ssl = no