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.cn/lpgeneau/slapos/-/raw/d23e3f3b0040d374d131037e73b07c7dd7db0ad3/software/seleniumserver/test/data/test.TestChrome69.test_screenshot.png)
![software/seleniumserver/test/data/test.TestChrome91.test_screenshot.png](https://lab.nexedi.cn/lpgeneau/slapos/-/raw/d23e3f3b0040d374d131037e73b07c7dd7db0ad3/software/seleniumserver/test/data/test.TestChrome91.test_screenshot.png)