Is it important to consider channel bandwidth of the signal to measure cable path loss for RF testing?
If you view the cable as a transmission line, you can model all of its characteristics using standard transmission line formulas. This suggests that single frequency tests at the spectrum limits should provide most of the needed information.

Take care however, that your tests
also examine group delays given the wide spectrum of the application. With careful phase change measurements across the application spectrum, you will be able to predict any possible group delay effects.