It is up to you.
Just pick one Track and stay on it or take one Tight-, one Flowing- and one Highspeed-Track for those Testings.
Personally I like:
Ascari -- Good selection of different Turns and decent straights
Nordschleife -- Bumpy roads great corner variations but a bit long
Suzuka -- Plain roads and has all turns you need and decent straights
Madrid -- Tight City-Curse with long straight and variable Corners
Laguna Seca -- Just because its short and it has a good amount of different Turns plus some have a banking and it has !! THE CHICANE !! where bad setups will be exposed
Grand Valley -- One of my best known Tracks just because it is in the Series since beginning

Great Test-Track
Apricot Hill -- Short and has the same benefits like Laguna but without the Corkscrew

In addition decent main straight.
Others I personally drive for a few tests:
Tokyo, Trial Mountain, Deep Forest, Daytona Road, Nürb GP (both), Monza (SPEED), Brands Hatch (both) and Silverstone
As said stay with any choice you make and drive various Cars there before you want to compare certain cars at certain pp-values.
So your confident with the Track/Tracks.