Test esplora example

[vladimirfomene]

This PR implements basic test for bdk_esplora_example. The following test are implement in this PR:

• Test for confirmed balance
• Test for unconfirmed balance
• Test for Reorg
• Test for sending a tx

I had to implement a workaround for the reorg test to work for Esplora because after a reorg on bitcoind, electrs still shows the transactions in reorged blocks as confirmed. I followed @notmandatory answer to this stack exchange question

[codecov-commenter]

Codecov Report

Merging #195 (40a89de) into master (2a11787) will not change coverage. The diff coverage is n/a.

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@@           Coverage Diff           @@
##           master     #195   +/-   ##
=======================================
  Coverage   56.74%   56.74%           
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  Files          10       10           
  Lines         252      252           
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  Hits          143      143           
  Misses        109      109           

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