[fix] ffi/framebuffer: memory use explosion in unit tests (#508)
[fix] ffi/framebuffer: memory use explosion in unit tests (#508)

During the unit tests fb:getDPI() is hit multiple times and self.dpi has to be filled from scratch.
This results in multiple calls to `os.getenv("EMULATE_READER_DPI")`. No biggie you would think, but
for some reason this causes memory use to quickly balloon out of proportion and exceed the 2 GB limit.
houqp/koreader-base,NiLuJe/koreader-base,apletnev/koreader-base,Frenzie/koreader-base,koreader/koreader-base,apletnev/koreader-base,koreader/koreader-base,NiLuJe/koreader-base,apletnev/koreader-base,koreader/koreader-base,Frenzie/koreader-base,NiLuJe/koreader-base,Frenzie/koreader-base,houqp/koreader-base,houqp/koreader-base,NiLuJe/koreader-base,apletnev/koreader-base,Frenzie/koreader-base,houqp/koreader-base,koreader/koreader-base