Test execution wall clock is simply a way of referring to the amount of time test execution takes/took. They key point point of using the term being to ensure there's no ambiguity that they want the total time (including time that the CPU is not active, e.g. waiting time built into your tests) included in the time taken to execute.
In the case of the sample question you gave: 3) For the test cases you’ve written, what is the wall clock? How frequently did the test cases run?
What they're trying to figure out here seems to be, how quickly have your tests executed in the past? The frequency part of the question indicates that what they're interested in here (or probably trying to achieve) is a suite of automated tests that are executed with builds and run quickly - their goal is probably highly effective and fast continuous integration with automated testing built in.
(Although perhaps it could also be read as how long it took you to write the tests rather than execute them)