I'm guessing we are talking about standard RC parts here so, the servo tester most likely outputs a signal of 20ms period. The width of the pulse within this 20ms period determines the output of the ESC. Actual values and their relation to each other depend on the hardware used.

Can the ESC be driven from a 0 to 10v signal? Not directly. I'm sure there'll be something on the web to suit or maybe this is another job for a friendly PIC.