Which tracks are best for testing cars? I need to have a proper full time testing track to help me choose what cars I use for online or career - I can then compare cars with similar PP stats!

Depends on what you want to test...
If you are playing online on some particular tracks only, then test on those tracks as well.
If you want to compare cars' laptimes vs PP (which car is fastest for given PP), then I'd suggest:
Fuji (high speed corners, low speed corners and long straight for acceleration)
Nurburgring GP course (low and mid speed corners and good test of suspension, as the track is bumpy in some places)
Silverstone (low, mid, high speed corners, easy track on driver mistakes)
Some tips for choosing track:
- Don't use tracks which are hard, as the lap time will be influenced by your (lack-of) skill and not car's performance
- Don't use very short and slow tracks (a.k.a Tsukuba)
- Don't use very long tracks (Spa, Nurburgring) because it's hard to get perfect consistency from the driver
- Don't use High speed only tracks (Monza, Indy oval) because corners are more important than straights