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