test: test_xpay: add test for 1sat invoice #14959

Job	Run time
Pre-build checks	17s
Compile CLN ${{ matrix.cfg }}	0s
Test CLN ${{ matrix.name }}	0s
Run fuzz regression tests	0s
Sanitizers Test CLN	0s
Run unit tests	0s
Test minimum supported BTC v${{ matrix.MIN_BTC_VERSION }} with ${{ matrix.NAME }}	0s
Valgrind Test CLN ${{ matrix.name }}	0s
CI completion	2s
Check we can downgrade the node	0s
Update examples in doc schemas (disabled temporarily!)	0s
	19s

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