add tests for downloading and fix retry bug

[poszu]

Fixed 2 bugs in the downloading code:

1. Expect a 206 (Partial Content) HTTP status code and retry on any different code
2. Use the persisted redirect URL when retrying in download_with_retries. Previously, it could happen that the server changes the DB in between tries which led to partially downloading versions A and B.

[codecov]

Codecov Report

Attention: Patch coverage is 69.69697% with 10 lines in your changes missing coverage. Please review.

Project coverage is 48.03%. Comparing base (084107c) to head (f5723de).
Report is 7 commits behind head on main.

Files	Patch %	Lines
src/go_spacemesh.rs	0.00%	4 Missing ⚠️
src/utils.rs	0.00%	4 Missing ⚠️
src/download.rs	92.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #32       +/-   ##
===========================================
+ Coverage   24.80%   48.03%   +23.22%     
===========================================
  Files          11       11               
  Lines         254      254               
===========================================
+ Hits           63      122       +59     
+ Misses        191      132       -59     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[brusherru]

👍