Update source-generators.cookbook.md for recommended testing approach (#51725)
Update source-generators.cookbook.md for recommended testing approach (#51725)


CyrusNajmabadi/roslyn,shyamnamboodiripad/roslyn,diryboy/roslyn,bartdesmet/roslyn,diryboy/roslyn,mavasani/roslyn,sharwell/roslyn,wvdd007/roslyn,weltkante/roslyn,eriawan/roslyn,KevinRansom/roslyn,sharwell/roslyn,dotnet/roslyn,physhi/roslyn,mavasani/roslyn,wvdd007/roslyn,jasonmalinowski/roslyn,CyrusNajmabadi/roslyn,wvdd007/roslyn,KevinRansom/roslyn,sharwell/roslyn,KevinRansom/roslyn,bartdesmet/roslyn,dotnet/roslyn,shyamnamboodiripad/roslyn,physhi/roslyn,CyrusNajmabadi/roslyn,jasonmalinowski/roslyn,physhi/roslyn,eriawan/roslyn,diryboy/roslyn,weltkante/roslyn,weltkante/roslyn,dotnet/roslyn,jasonmalinowski/roslyn,mavasani/roslyn,bartdesmet/roslyn,eriawan/roslyn,shyamnamboodiripad/roslyn