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.
hide outer repo
$ hg init
$ echo '[Section]' >> $HGRCPATH
$ echo 'KeY = Case Sensitive' >> $HGRCPATH
$ echo 'key = lower case' >> $HGRCPATH
$ hg showconfig Section
Section.KeY=Case Sensitive
Section.key=lower case