1. A system, comprising: one or more processing units; and a memory coupled to the one or more processing units, the memory storing software modules for execution by the one or more processing units, the software modules comprising: a runtime engine that is configured to execute a script, to collect profile data concerning the script during execution thereof, and to perform just-in-time (JIT) compilation of selected portions of the script based on the profile data to obtain efficient execution thereof in accordance with an optimization; and a profile data mutator that is configured to: modify one or more profile data entities within the profile data to facilitate testing of the runtime engine; and test the runtime engine by performing at least changing a value of each profile data entity from a first valid value to a second randomly-determined valid value, the runtime engine being configured to execute the script in accordance with the second randomly-determined valid value to obtain efficient execution thereof based on a previously-untested optimization.