fix(test): use standalone aws-account-map v1.537.2 for terratest

Terratest was failing with the same cloudposse/utils constraint conflict
that previously affected CI Lint:

    no available releases match the given constraints
    != 1.4.0, >= 1.7.1, >= 2.0.0, < 2.0.0, < 3.0.0

When atmos applies aurora-postgres/basic and aurora-postgres/serverless,
src/providers.tf references ../account-map/modules/iam-roles as a local
module. That module's dependency graph therefore merges into aurora-postgres's
provider constraints. The test was vendoring account-map from the legacy
monorepo (cloudposse/terraform-aws-components//modules/account-map at
1.520.0), whose iam-roles/main.tf still uses
cloudposse/stack-config/yaml//modules/remote-state v1.5.0 — which pins
cloudposse/utils >= 1.7.1, < 2.0.0.

The previous commit (b6bfb7b) already bumped src/remote-state.tf to
stack-config v2.0.0 (utils >= 2.0.0, < 3.0.0), so the two constraints
were irreconcilable at terraform init.

Switching the vendor source to the standalone
cloudposse-terraform-components/aws-account-map repo at v1.537.2 brings
the iam-roles sub-module up to stack-config v2.0.0, aligning all
cloudposse/utils constraints in the init graph to >= 2.0.0, < 3.0.0.

Validated locally: `atmos vendor pull` + `terraform init -backend=false`
in a simulated aurora-postgres working dir resolves cloudposse/utils
v2.5.0 cleanly with no constraint conflicts.

Author: ivan-pinatti

test/fixtures/vendor.yaml

@@ -6,8 +6,8 @@ metadata:
 spec:
   sources:
     - component: "account-map"
-      source: github.com/cloudposse/terraform-aws-components.git//modules/account-map?ref={{.Version}}
-      version: 1.520.0
+      source: github.com/cloudposse-terraform-components/aws-account-map.git//src?ref={{.Version}}
+      version: v1.537.2
       targets:
         - "components/terraform/account-map"
       included_paths: