TestConfig.StringAccessors seems to be flaky

TestConfig.StringAccessors seems to be flaky, or at least fragile. I got a test failure in an unrelated PR recently:

[2022-10-31 13:02:05.770][13641667][critical][assert] [external/envoy/test/test_listener.cc:21] assert failure: !Thread::MainThread::isMainThreadActive(). Details: MainThreadLeak: [TestConfig.StringAccessors] test exited before main thread shut down

https://github.com/envoyproxy/envoy-mobile/actions/runs/3322946043/jobs/5571794275

Doing some local testing, I noticed that if I re-order the tests in envoy_config_test.cc so that this test does not run last, then it causes failures reliably.