Extend by two if the side to move has only one legal move

[TarasVuk]

This patch returns mate score instead of alpha in singular search.

Passed STC:
LLR: 2.97 (-2.94,2.94) <-1.75,0.25>
Total: 56576 W: 14374 L: 14172 D: 28030
Ptnml(0-2): 171, 6646, 14472, 6808, 191
https://tests.stockfishchess.org/tests/view/6564e3ab136acbc57354d443

Passed LTC:
LLR: 3.13 (-2.94,2.94) <-1.75,0.25>
Total: 103782 W: 25524 L: 25370 D: 52888
Ptnml(0-2): 58, 11753, 28118, 11901, 61
https://tests.stockfishchess.org/tests/view/6565e4c8136acbc57354ed23

bench: 1451915

[Disservin]

I think this is a very hacky way and since it's a functional change and logic change, non regression bounds are not enough. It should also then be done differently instead of letting bestValue become a mate value.