test: add known broken test for duplicate thread-id terms

According to my bijection, this bug has been present since commit
411675a6ce in 2017. It is not completely clear what harm it causes in
regulary use, but it (at least) makes notmuch crash when compiled with
-DDEBUG_DATABASE_SANITY.

test/T670-duplicate-mid.sh

@@ -2,10 +2,26 @@
 test_description="duplicate message ids"
 . $(dirname "$0")/test-lib.sh || exit 1
 
+test_require_external_prereq xapian-delve
+
 add_message '[id]="duplicate"' '[subject]="message 1" [filename]=copy1'
 add_message '[id]="duplicate"' '[subject]="message 2" [filename]=copy2'
 
 add_message '[id]="duplicate"' '[subject]="message 0" [filename]=copy0'
+
+test_begin_subtest 'at most 1 thread-id per xapian document'
+test_subtest_known_broken
+db=${MAIL_DIR}/.notmuch/xapian
+for doc in $(xapian-delve -1 -t '' "$db" | grep '^[1-9]'); do
+    xapian-delve -1 -r "$doc" "$db" | grep -c '^G'
+done > OUTPUT.raw
+sort -u < OUTPUT.raw > OUTPUT
+cat <<EOF > EXPECTED
+0
+1
+EOF
+test_expect_equal_file EXPECTED OUTPUT
+
 test_begin_subtest 'search: first indexed subject preserved'
 cat <<EOF > EXPECTED
 thread:XXX   2001-01-05 [1/1(3)] Notmuch Test Suite; message 1 (inbox unread)