[Test] Extend test_slurm_accounting to use a mixed-case ClusterName

[gmarciani]

Description of changes

Extend test_slurm_accounting to use a mixed-case ClusterName via CustomSlurmSettings, protecting against regressions fixed in aws/aws-parallelcluster-cookbook#3164 and also reported in aws/aws-parallelcluster-cookbook#3159

Tests

SUCCEEDED end2end test:

test-suites:
  schedulers:
    test_slurm_accounting.py::test_slurm_accounting:
      dimensions:
        - regions: ["us-east-1"]
          instances: ["c5.xlarge"]
          oss: ["alinux2023"]
          schedulers: ["slurm"]

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