Add a helper function for handling locally cached image data (PR 18269 follow-up)

[Snuffleupagus]

This avoids having to duplicate the same exact code multiple times.

[Snuffleupagus]

/botio test

[moz-tools-bot]

From: Bot.io (Linux m4)

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Received

Command cmd_test from @Snuffleupagus received. Current queue size: 0

Live output at: http://54.241.84.105:8877/1488059b7a5d2f4/output.txt

[moz-tools-bot]

From: Bot.io (Windows)

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Received

Command cmd_test from @Snuffleupagus received. Current queue size: 0

Live output at: http://54.193.163.58:8877/e455565ab95bab4/output.txt

[timvandermeij]

r=me, with passing tests. Thank you for deduplicating this!

[moz-tools-bot]

From: Bot.io (Linux m4)

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Failed

Full output at http://54.241.84.105:8877/1488059b7a5d2f4/output.txt

Total script time: 28.79 mins

• Unit tests: Passed
• Integration Tests: FAILED
• Regression tests: FAILED

  different ref/snapshot: 18
  different first/second rendering: 1

Image differences available at: http://54.241.84.105:8877/1488059b7a5d2f4/reftest-analyzer.html#web=eq.log

[moz-tools-bot]

From: Bot.io (Windows)

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Failed

Full output at http://54.193.163.58:8877/e455565ab95bab4/output.txt

Total script time: 43.18 mins

• Unit tests: FAILED
• Integration Tests: FAILED
• Regression tests: FAILED

  different ref/snapshot: 5
  different first/second rendering: 1

Image differences available at: http://54.193.163.58:8877/e455565ab95bab4/reftest-analyzer.html#web=eq.log

[timvandermeij]

I have created #18276 for the new intermittently failing integration test. It's not related to this PR, but I don't recall having seen this particular one before so I'm just creating issues for every single one I find in the hope that soon we'll get rid of all of them :-)