test-largefiles.t: fix wrong '#if hgweb' - it should be '#if serve'
A bug introduced in
0c1d10351869 did that a part of the test never was run.
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state