test_attach_tenant_config: add allowed error

[koivunej]

evidence of quite rare flaky. the detach can cause this with the right timing.

[github-actions]

3144 tests run: 3017 passed, 0 failed, 127 skipped (full report)

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Flaky tests (6)

Postgres 16

• test_tenant_import[4-local_fs]: debug

Postgres 14

• test_import_from_pageserver_small: debug
• test_empty_branch_remote_storage_upload: debug
• test_synthetic_size_while_deleting: debug
• test_vm_bit_clear_on_heap_lock: debug
• test_pageserver_lsn_wait_error_safekeeper_stop: debug

Code coverage* (full report)

• functions: 31.3% (6414 of 20481 functions)
• lines: 48.0% (49312 of 102647 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
22aa514 at 2024-05-22T16:25:28.453Z :recycle:}