roachtest: add end-to-end test for token-based CPU AC

[tbg]

We should add a roachtest that performs an end-to-end verification of the behavior of the token-based CPU AC system:

• tenant should be able to consume significant amount of CPU (~80%) when system otherwise idle.
• tenant should not be able to be a "noisy neighbor", i.e. when multiple tenants request significant amount of CPU, they should share fairly, and not drive system as a whole above 90% CPU utilization.
• tenant that performs only a short burst of work should be preferred over longer-running noisy tenants.
• foreground latencies, esp. in the tail, for system tenant or low-CPU tenant should remain under control.

Epic: CRDB-54929

Jira issue: CRDB-55039

[tbg]

potentially related: #137027