context: add test for the scenario that Context's logger itself is using Context

This will break if
7f1ac34d,
2f6e2c87,
a3a54209
are reverted:

```
io.grpc.ContextTest > initContextWithCustomClassLoaderWithCustomLogger FAILED
    java.lang.ExceptionInInitializerError
        at io.grpc.ContextTest$LoadMeWithStaticTestingClassLoader.run(ContextTest.java:789)
        at io.grpc.ContextTest.initContextWithCustomClassLoaderWithCustomLogger(ContextTest.java:760)

        Caused by:
        java.lang.RuntimeException: Storage override had failed to initialize
            at io.grpc.Context.storage(Context.java:134)
            at io.grpc.Context.current(Context.java:163)
            at io.grpc.ContextTest$LoadMeWithStaticTestingClassLoader$1.publish(ContextTest.java:773)
            at java.util.logging.Logger.log(Logger.java:738)
            at java.util.logging.Logger.doLog(Logger.java:765)
            at java.util.logging.Logger.log(Logger.java:875)
            at io.grpc.Context.<clinit>(Context.java:122)
            ... 2 more
```