tests/data/: #bdb,hdb# -> #maindb/indexdb#.

Done for keywords index,limits.  (Also do 'mode' if there were any cases.)
Change generated by:  perl -i -pw0777e '
  INIT { $q=qr/\#(?:[bh]|index|main)db\#/im; $k=qr/(?:index|limits|mode)\b/ }
  0 while s/^$q($k.*\n)((?:$q.*\n)*?)\n?$q\1/\#maindb\#$1$2/imo;
  s/^\#maindb\#index/\#indexdb\#index/igm;
' tests/data/*.* tests/data/regressions/*/*.*