[Test] Better workaround for filelists.swift
bdf60152f097d2901d134300ae69019a9d0d4ef2 added some extra padding to
forFilelistCapture to avoid it being exactly 4096 bytes, triggering a
tail bug. Rather than do that, just run the sed's first so that the
input is quite small (definitely under 4096 bytes).
Resolves rdar://105395733.
