Merge #9909: tests: Add FindEarliestAtLeast test for edge cases

Summary:
cf66f04 Add FindEarliestAtLeast test for edge cases (Russell Yanofsky)

Tree-SHA512: bed2e9b2eefe0014414f1be8f0ba236b1dcc6744a670ffa546100c62817dd06a117cc62cdff80c4988d2b374cae9a57f2bb135fc582ffb51ca2d34ab07bb3fe0

Backport of Core PR9909
https://github.com/bitcoin/bitcoin/pull/9909/

Test Plan:

make check

Reviewers: jasonbcox, deadalnix, Fabien, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: Fabien, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3244