Testing is done alone, with no official scoring of times. Laps are timed within the simulation, but the results of testing are unofficial and are not saved or published to the website as other results are.
Incidents do not count against you in testing.
You can test any track and car you have on your account by pressing the TEST button on the navigation bar
or you can test the current series car and track for the race week by opening the desired series, then click the down arrow next to "More ways to race" then "Test Drive:
Testing is not an official session. You can test whenever you like for as long as you like.