refactor: use assert in remaining gateway tests

[hacdias]

Just use the assert library more thoroughly.

[codecov]

Codecov Report

Merging #183 (c60aa9c) into main (7af2b2b) will decrease coverage by 0.11%.
The diff coverage is n/a.

❗ Current head c60aa9c differs from pull request most recent head 7fca823. Consider uploading reports for the commit 7fca823 to get more accurate results

@@            Coverage Diff             @@
##             main     #183      +/-   ##
==========================================
- Coverage   29.67%   29.57%   -0.11%     
==========================================
  Files         100      100              
  Lines       11258    11241      -17     
==========================================
- Hits         3341     3324      -17     
+ Misses       7550     7547       -3     
- Partials      367      370       +3     

Impacted Files	Coverage Δ
gateway/handler.go	62.75% <0.00%> (-1.48%)	⬇️
gateway/handler_unixfs__redirects.go	38.79% <0.00%> (-0.55%)	⬇️
gateway/handler_tar.go	0.00% <0.00%> (ø)
gateway/handler_block.go	0.00% <0.00%> (ø)
gateway/handler_codec.go	0.00% <0.00%> (ø)
gateway/handler_unixfs_dir.go	65.10% <0.00%> (ø)
...tswap/client/internal/session/sessionwantsender.go	0.00% <0.00%> (ø)
files/multifilereader.go	82.02% <0.00%> (+0.20%)	⬆️
gateway/handler_unixfs.go	66.66% <0.00%> (+2.66%)	⬆️

[hacdias]

I'm not exactly sure how the coverage decreased, and there is no coverage diff. It doesn't make sense to me since I did not change the tests themselves.