chore(tests): mark flaky tests

[jansegre]

These tests were added recently and they both use the simulator in a way that will sometimes fail, which has nothing to do with what is being tested. I think it makes sense to have them as flaky and avoid having to re-run the failed jobs when it occasionally happens.

[codecov]

Codecov Report

Merging #400 (88764be) into dev (9b1c3b6) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##              dev     #400      +/-   ##
==========================================
- Coverage   82.84%   82.82%   -0.02%     
==========================================
  Files         171      171              
  Lines       15585    15585              
  Branches     2466     2466              
==========================================
- Hits        12912    12909       -3     
- Misses       2227     2228       +1     
- Partials      446      448       +2     

Impacted Files	Coverage Δ
hathor/indexes/manager.py	92.85% <0.00%> (-0.90%)	⬇️
hathor/p2p/node_sync.py	83.53% <0.00%> (-0.50%)	⬇️

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