Test utils: replaced the `timeit()` method with `timeNow()` and `elapsedTime()`.
Test utils: replaced the `timeit()` method with `timeNow()` and `elapsedTime()`.

- The `timeit()` method uses a method that is not portable while we now rely on C++11 to time things accurately.
- We don't have one method only anymore since this required passing a very specific type of function while we should be allowed to time anything we want.

hsorby/libcellml,hsorby/libcellml,cellml/libcellml,cellml/libcellml,cellml/libcellml,cellml/libcellml,nickerso/libcellml,nickerso/libcellml,nickerso/libcellml,hsorby/libcellml,hsorby/libcellml,nickerso/libcellml