tests: Non-stopped simulator causing other tests to fail

[msbrogli]

Fixes #574.

Acceptance criteria

1. Stop simulator on tearDown().

[codecov]

Codecov Report

Merging #578 (cd96635) into master (17ebc11) will increase coverage by 0.04%.
The diff coverage is 95.49%.

@@            Coverage Diff             @@
##           master     #578      +/-   ##
==========================================
+ Coverage   83.41%   83.46%   +0.04%     
==========================================
  Files         215      221       +6     
  Lines       18986    19101     +115     
  Branches     2613     2618       +5     
==========================================
+ Hits        15838    15943     +105     
- Misses       2576     2581       +5     
- Partials      572      577       +5     

Impacted Files	Coverage Δ
hathor/conf/unittests.py	100.00% <ø> (ø)
hathor/transaction/transaction_metadata.py	90.69% <50.00%> (-0.97%)	⬇️
hathor/manager.py	70.84% <75.00%> (+0.11%)	⬆️
hathor/simulator/simulator.py	93.15% <80.00%> (-1.01%)	⬇️
hathor/consensus/block_consensus.py	94.34% <94.34%> (ø)
hathor/wallet/base_wallet.py	95.38% <94.89%> (-0.49%)	⬇️
hathor/consensus/transaction_consensus.py	96.11% <96.11%> (ø)
hathor/conf/settings.py	100.00% <100.00%> (ø)
hathor/consensus/__init__.py	100.00% <100.00%> (ø)
hathor/consensus/consensus.py	100.00% <100.00%> (ø)
... and 7 more

... and 1 file with indirect coverage changes