tracker calls fn exact

• fn {Function} Default A no-op function.
• exact {number} Default 1.
• Returns: {Function} that wraps fn.

The wrapper function is expected to be called exactly exact times. If the function has not been called exactly exact times when [tracker.verify()][] is called, then [tracker.verify()][] will throw an error.

const assert = require('assert');
// Creates call tracker.
const tracker = new assert.CallTracker();
function func() {}
// Returns a function that wraps func() that must be called exact times
// before tracker.verify().
const callsfunc = tracker.calls(func);