test: Fix race in firefox CDP addScriptToEvaluateOnNewDocument() emulation
test: Fix race in firefox CDP addScriptToEvaluateOnNewDocument() emulation

Latest firefox now often creates short-lived internal frames which
come, immediately go, and get replaced with a non-null real frame:

```
CDP: executionContextCreated {"context":{"id":1,"origin":"null","name":"","auxData":{"isDefault":true,"frameId":"19","type":"default"}}}
<- {'frameId': '19', 'loaderId': '505515503255631'}
-> expect_load
CDP: executionContextDestroyed 1
CDP: frameNavigated {"frame":{"id":"19","loaderId":"505515503255631","url":"http://127.0.0.2:9091/system","securityOrigin":null,"mimeType":null}}
CDP: executionContextCreated {"context":{"id":2,"origin":"http://127.0.0.2:9091","name":"","auxData":{"isDefault":true,"frameId":"19","type":"default"}}}

```

Our addScriptToEvaluateOnNewDocument() trips over this, as injecting our
scripts will fail the Runtime.evaluate() promise with a "Cannot find
context" error. Ignore this particular error, but log it to ease
debugging.

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