[funfuzz] Test whether Linux system is able to generate core dumps. If not, run jsfunfuzz within gdb instead

[nth10sd]

Ref https://github.com/MozillaSecurity/funfuzz/blob/1fc970fe0643a32a9aae4fa0d138ec11fbb53e8d/src/funfuzz/js/js_interesting.py#L133 we should have a mode to test whether the local Linux system is able to generate core dumps, early on. (i.e. whether ulimit -c unlimited is set and working properly)

If it cannot, we should run jsfunfuzz or the intended testcase within a debugger instead.

[nth10sd]

If it can, we might want to randomly consider this mode (with gdb) and without gdb (normal) and possibly with rr.