test: make gdb even quieter

gdb sometimes writes warnings to stdout, which we don't need/want, and
for some reason --batch-silent isn't enough to hide. So in this commit
we write them to a log file, which is probably better for debugging
anyway. To see an illustrative test failure before this change, run

% make
% touch notmuch-count.c
% cd test && ./T060-count.sh

(cherry picked from commit f45fa5bdd3)

test/T060-count.sh

@@ -103,6 +103,8 @@ restore_database
 
 cat <<EOF > count-files.gdb
 set breakpoint pending on
+set logging file count-files-gdb.log
+set logging on
 break count_files
 commands
 shell cp /dev/null ${MAIL_DIR}/.notmuch/xapian/postlist.${db_ending}

test/T070-insert.sh

@@ -192,6 +192,8 @@ for code in OUT_OF_MEMORY XAPIAN_EXCEPTION FILE_NOT_EMAIL \
 gen_insert_msg
 cat <<EOF > index-file-$code.gdb
 set breakpoint pending on
+set logging file index-file-$code.log
+set logging on
 break notmuch_database_add_message
 commands
 return NOTMUCH_STATUS_$code