Add tests for thread-safe slot mode

[gbrail]

Since we made "thread safe objects" an option a while back, we have very little testing of this feature, and @nabacg recently found a bunch of problems with the implementation. We should (in increasing order of difficulty):

1. Set up Test262SuiteTests to run the tests with this flag set
2. If we (and @rbri ) can get feature flag support set up, we can make this a feature flag too
3. Once we do that, we can run all the tests with this flag set

Now, since we already have a bunch of tests that take quite a while to run, I think that whatever we do should be an optional flag that we turn on only in CI.

[andreabergia]

I would at least recommend to run test262 with the thread-safety feature turned on, especially after #1754