[Test] Fix test 'test_update_slurm' adapting the constraint in the sbatch command to the flexible instance types used in the config

[gmarciani]

Description of changes

Fix test 'test_update_slurm' adapting the constraint in the sbatch command so that it can work when flexibleinstance types are used. In 1360e74 we introduced the use of flexible instance types in the test to reduce the risk of ICEs, but the sbatch command should have been adapted accordingly. Notiuce that you cannot use instance typoes as constraints when flexible instance types are used because in that case, the instance type is not listed in CR features.

Tests

1. Succeeded test_update_slurm.

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