Investigate and fix test failures in amp-ad type=custom

[rileyajones]

Description

The beforeEach block of test/integration/test-amp-ad-type-custom.js has started flaking. Rather than try to fix the issue I am electing to skip the test (there is only one) for now.

Note that this is only happening in Safari

Reproduction Steps

Run remove the skip and the tests in test/integration/test-amp-ad-type-custom.js
amp integration --report --nobuild --minified --safari

Relevant Logs

[20:52:26] Done. Running tests...
○○○○●●●●●●●●●●●●●●●
  amp-ad type=custom
    ✗ "before each" hook for "should render template"
	Timeout of 10000ms exceeded. For async tests and hooks, ensure "done()" is called; if returning a Promise, ensure it resolves.

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Safari 13.1.1 (Mac OS 10.15.5): Executed 69 of 535 (1 FAILED) (skipped 466) (22.288 secs / 15.913 secs)
TOTAL: 1 FAILED, 68 SUCCESS

[20:52:49] Stopped server at http://localhost:8081
[20:52:50] ERROR: Karma test failed with exit code 1
[20:52:50] 'integration' errored after 58s
[20:52:50] Error: Karma test failed with exit code 1
    at Runner.teardown (/Users/distiller/project/build-system/tasks/runtime-test/runtime-test-base.js:347:13)
    at runMicrotasks (<anonymous>)
    at processTicksAndRejections (node:internal/process/task_queues:96:5)
    at async integration (/Users/distiller/project/build-system/tasks/integration.js:39:3)
    at async runTask (/Users/distiller/project/build-system/task-runner/amp-task-runner.js:94:5)
    at async Command.<anonymous> (/Users/distiller/project/build-system/task-runner/amp-task-runner.js:231:7)
[20:52:50] ERROR: Integration tests failed!

Browser(s) Affected

No response

OS(s) Affected

No response

Device(s) Affected

No response

AMP Version Affected

No response

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed in 7 days if no further activity occurs. Thank you for your contributions.