software/seleniumserver,seleniumrunner: workaround firefox >= 60 font issue
This makes firefox 60 and 68 able to load some fonts and fixes the issue that fonts were rendered as garbled text
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![software/seleniumserver/test/data/test.TestFirefox60.test_screenshot.png](https://lab.node.vifib.com/nexedi/slapos/-/raw/ba527b82003c9cfdcc5a675b48588f4b59081bad/software/seleniumserver/test/data/test.TestFirefox60.test_screenshot.png)
This makes firefox 60 and 68 able to load some fonts and fixes the issue that fonts were rendered as garbled text
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