[TEST] Make it possible to customize whether the REST tests are run by default or not

We currently run REST tests by default. There might be third parties that use our test infra and don't want to run REST tests by default, especially since they need some additional configuration for spec and tests that are not shipped with the tests jar file. In those cases we might want to customize the value for the property that controls whether REST tests are run by default or not.

This change makes it possible to change the default value from a static block like the following:

```
static {
    ElasticsearchRestTests.REST_TESTS_ENABLED_BY_DEFAULT = false;
}
```