[bugfix] check bestmove only for multipv 1 lines

[robertnurnberg]

Fixes #998.

Master:

> ./fastchess -engine name=sf-dev1 cmd=./stockfish option.'MultiPV'=2 -engine name=sf-dev2 cmd=./stockfish option.'Skill Level'=10 -each tc=0.2+0.01 -rounds 5 -concurrency -1 -openings file=UHO_Lichess_4852_v1.epd format=epd > fc_master.txt
> grep Warning fc_master.txt | grep -c sf-dev1 
573
> grep Warning fc_master.txt | grep -c sf-dev2 
215

Patch:

> ./fastchess -engine name=sf-dev1 cmd=./stockfish option.'MultiPV'=2 -engine name=sf-dev2 cmd=./stockfish option.'Skill Level'=10 -each tc=0.2+0.01 -rounds 5 -concurrency -1 -openings file=UHO_Lichess_4852_v1.epd format=epd > fc_patch.txt
> grep Warning fc_patch.txt | grep -c sf-dev1
0
> grep Warning fc_patch.txt | grep -c sf-dev2
259

So the output shows that both correctly report bestmove mismatch for sf-dev with skill levels, while only master reports wrong bestmove for sf-dev in multiPV mode.