Smooth out doDeeperSearch

Smooth out doDeeperSearch

Adjust threshold based on the difference between newDepth and LMR depth.
With more reduction, bigger fail-high is required in order to perform the deeper search.

STC:
LLR: 2.96 (-2.94,2.94) <0.00,2.50>
Total: 93576 W: 24133 L: 23758 D: 45685
Ptnml(0-2): 260, 10493, 24935, 10812, 288
https://tests.stockfishchess.org/tests/view/61cbb5cee68b2a714b6eaf09

LTC:
LLR: 2.94 (-2.94,2.94) <0.50,3.00>
Total: 109280 W: 28198 L: 27754 D: 53328
Ptnml(0-2): 60, 11225, 31637, 11647, 71
https://tests.stockfishchess.org/tests/view/61cc03fee68b2a714b6ec091

closes https://github.com/official-stockfish/Stockfish/pull/3877

Bench: 4464723