chore: use go-fixtureplate to make unixfs fixtures

[rvagg]

Depends on release of https://github.com/ipld/go-fixtureplate and merge of ipfs/go-unixfsnode#53

This is not a huge value-add other than being a bit more explicit about what we're testing. I recently simplified this code anyway, this takes it a few steps more and uses the go-fixtureplate DSL to generate the synthetic UnixFS data. It's been useful in getting go-fixtureplate working nicely and the API cleaned up.

[codecov-commenter]

Codecov Report

Merging #436 (a71241d) into main (a9ecb49) will decrease coverage by 0.08%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #436      +/-   ##
==========================================
- Coverage   76.60%   76.53%   -0.08%     
==========================================
  Files          85       85              
  Lines        6387     6387              
==========================================
- Hits         4893     4888       -5     
- Misses       1242     1245       +3     
- Partials      252      254       +2     

Files	Coverage Δ
pkg/internal/itest/util.go	100.00% <100.00%> (ø)

... and 6 files with indirect coverage changes

[rvagg]

updated other deps while in there because we have a new storetheindex release that we can jump to from the untagged one we've been on; so this updates libipni and graphsync too.