serverutils: change allow_unsafe_internals default value to true for tests #155367
Description
The stable sql interface project involves the restricting of access to the crdb_internal and system namespaces to external callers. This project is being done in two phases, the first where the ability to restrict access is available for testing, and then the second where these interfaces are actually restricted by default.
In order to accomplish the second part of this process, we need to allow all testing surfaces to succeed, which includes a number of things which attempt to read the unsafe internals. You can see examples of this breakage (and some of their possible fixes) in this PR. Since the tests are so heavily dependent on access to internal structures, part of this effort will be to allow all tests to access the unsafe internals. We'll do this likely by backing the tests with a cluster setting for the default session variable value, and setting it to true for tests.
Epic: CRDB-55276
Jira issue: CRDB-55410