AEA-699 connections test coverage

[solarw]

Proposed changes

better connections coverage

Fixes

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have read the CONTRIBUTING doc
• I am making a pull request against the develop branch (left side). Also you should start your branch off our develop.
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have checked that code coverage does not decrease.
• I have checked that the documentation about the aea cli tool works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in downstream modules

Further comments

[codecov-commenter]

Codecov Report

Merging #1488 into develop will increase coverage by 1.28%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #1488      +/-   ##
===========================================
+ Coverage    88.76%   90.05%   +1.28%     
===========================================
  Files          243      243              
  Lines        17178    17157      -21     
===========================================
+ Hits         15248    15450     +202     
+ Misses        1930     1707     -223     

Flag	Coverage Δ
#unittests	90.05% <100.00%> (+1.28%)	⬆️

Impacted Files	Coverage Δ
packages/fetchai/connections/ledger/connection.py	69.56% <ø> (+37.13%)	⬆️
.../fetchai/connections/ledger/contract_dispatcher.py	51.31% <ø> (+16.79%)	⬆️
...es/fetchai/connections/ledger/ledger_dispatcher.py	67.44% <ø> (+38.71%)	⬆️
packages/fetchai/connections/ledger/base.py	71.18% <100.00%> (+18.64%)	⬆️
aea/test_tools/test_cases.py	85.22% <0.00%> (-1.71%)	⬇️
packages/fetchai/protocols/ledger_api/dialogues.py	85.18% <0.00%> (+3.70%)	⬆️
...ckages/fetchai/protocols/contract_api/dialogues.py	92.85% <0.00%> (+10.71%)	⬆️
...ges/fetchai/protocols/contract_api/custom_types.py	77.41% <0.00%> (+12.90%)	⬆️
... and 4 more

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[DavidMinarsch]

LGTM, please fix failing static tests

[DavidMinarsch]

LGTM