software/seleniumserver: configure fontconfig
This was disabled to "keep compatibility with current font selection problems", but I don't think we need to care about compatibility for this. This was anyway causing a problem that system fontconfig will be used if it's available, so it's better to define things explictly to prevent falling back to system configuration.
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![software/seleniumserver/test/data/test.TestChrome69.test_screenshot.png](https://lab.nexedi.com/jerome/slapos/-/raw/84c504e290541e3741e78c4d827e33eea90ba8ac/software/seleniumserver/test/data/test.TestChrome69.test_screenshot.png)
![software/seleniumserver/test/data/test.TestChrome91.test_screenshot.png](https://lab.nexedi.com/jerome/slapos/-/raw/84c504e290541e3741e78c4d827e33eea90ba8ac/software/seleniumserver/test/data/test.TestChrome91.test_screenshot.png)