Simplify extension when ttMove is assumed to fail high over current beta

Simplify extension value to -3 when ttMove is assumed to fail high over current beta.

Passed non-reg STC:
https://tests.stockfishchess.org/tests/view/65d66ed81d8e83c78bfddcba
LLR: 2.95 (-2.94,2.94) <-1.75,0.25>
Total: 235136 W: 60711 L: 60708 D: 113717
Ptnml(0-2): 969, 27904, 59874, 27797, 1024

Passed non-reg LTC:
https://tests.stockfishchess.org/tests/view/65da2994944f2a78d4733107
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 662850 W: 166161 L: 166602 D: 330087
Ptnml(0-2): 394, 74895, 181274, 74482, 380

closes official-stockfish#5088

Bench: 1553115

src/search.cpp

@@ -1056,7 +1056,7 @@ Value Search::Worker::search(
 
                 // If the ttMove is assumed to fail high over current beta (~7 Elo)
                 else if (ttValue >= beta)
-                    extension = -2 - !PvNode;
+                    extension = -3;
 
                 // If we are on a cutNode but the ttMove is not assumed to fail high over current beta (~1 Elo)
                 else if (cutNode)