infra: consider adding test case that reproduces bug in #329

#329 mentions a bug where an old pack persists on the sound page for a sound with no pack, if users access that sound through a Similar soundlist for a sound that _does_ have a pack.

I think we should consider adding a test case to our suite that reproduces this bug, i.e., visits a packless sound through a soundlist of a sound that _does_ have a pack, and notice that the old pack persists.