fix(tests): unbreak CI against upstream Warehouse inventory-account validation

[kavin-114]

Summary

Unbreaks CI for every open PR. ERPNext develop (installed fresh by CI) added Warehouse.validate_inventory_account, which throws when a new Warehouse's company has no default inventory account. The tier2a permission fixture deliberately creates companies without a chart of accounts, so every shard containing test_tier2a_erpnext_reads now fails in setUpClass ("Please set Account in Warehouse ... or Default Inventory Account in Company ..."). Confirmed on two unrelated PRs (#886, #888) with identical signatures; the same heads were green yesterday.

One-line fix: the fixture sets the escape flag upstream provides for exactly this case (flags.ignore_inventory_account_validation), harmless on older ERPNext. Verified locally with the fixture-exercising class.

Pre-merge checklist

• CI is green — the tests check on this PR passes (never merge on ❌)
• Branch is up to date with develop
• New/changed behavior has tests (this IS a test fix)
• I self-reviewed the diff