A test was a discrete unit of a larger cricketing competition. Each test lasted for multiple days. The winner of a particular competition was the team that took the majority of the tests.
For instance, in the 1981 Ashes series, tests lasted at least five days each. By the third day of the third test, England were being given long, 500-1 odds to win the series because they were already one test down, and seemed to be flagging late in that third test. (PROSE: Graham Dilley Saves the World)