I’m not sure what value you have set as testVariable on the command line and why that needs to be parse that way. 
Also, at that point it would just be the string {{testVariable}} because that’s what you have set as the variable. You’d need to use .replaceIn() to resolve that value.
Using my example:
let non_resolved_var = pm.collectionVariables.get("testing");
let resolved_var = pm.variables.replaceIn(pm.collectionVariables.get("testing"));
console.log(non_resolved_var)
console.log(resolved_var)
Would display this if logged out the console:
