LibFuzzer -runs= option gets used in LibFuzzer::verify()

[ranweiler]

The LibFuzzer wrapper has a verify() method for validating that a fuzzer doesn't crash against a corpus.

In the libfuzzer_fuzz task, we invoke it here.

If target_options contains the Libfuzzer -runs=N argument, we will test each randomly-selected input N times. We should expect values of N ranging 10k (for very leaky targets), to much higher.

We stopped passing -runs=N to the non-fuzzing tasks in thelibfuzzer basic template in #1610, but this doesn't fix our re-use of target_options for both fuzzing and input-testing in the verify() stage of libfuzzer_fuzz.