Hi helmet
Our AMM says:
...
The test circuit is activated when the CVR TEST pushbutton switch on the overhead panel is pushed. The circuit consists of an oscillator producing a 600 Hz signal and a switching circuit which switches the oscillator output to each of the channels on the recording head in a sequence from 1 to 4 P.s.: A NOTE remark: it can be also a 800 HZ tone. Depends on the CVR PN.
So a test runs as long as you push the button and 600(or 800) Hz tone is produced and send to the channels.
HIH
Ingo