Makefile+workflows+sample-btcwallet.conf: Introduce sample conf check to CI workflow

[mohamedawnallah]

Change Description

Towards #1041.
Towards #1015.

Steps to Test

Steps for reviewers to follow to test the change.

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Testing

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• Tests covering the positive and negative (error paths) are included.
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[yyforyongyu]

LGTM👍

[GustavoStingelin]

tACK

Just a heads-up: if the script does not find the btcwallet binary, it will not fail.