test: Avoid using unreliable, hard-coded thread ID values in test suite.

test: Avoid using unreliable, hard-coded thread ID values in test suite.

Some recently-added tests used hard-coded thread ID values in search
specifications. This is unreliable since the thread IDs depend on the
order in which "notmuch new" encounters new files, (which in turn can
depend on inode ordering within the filesystem).

Fix these by using the new "notmuch search --output=threads" to find the
correct thread IDs given a hard-coded (but reliable) message ID.