dev tools: Fix regression introduced in commit "Print the test data to stdout for convenience.".

tee(1) truncated $fname.  Fortunately, the data that got truncated had just
been `git add`-ed, so no harm was done.

tests/generate.zsh

@@ -63,7 +63,7 @@ fi
 git add -- $fname
 
 # Buffer
-exec > >(tee $fname)
+exec > >(tee -a $fname)
 print -n 'BUFFER='
 if [[ $buffer != (#s)[$'\t -~']#(#e) ]]; then
   print -r -- ${(qqqq)buffer}