tracker-extract: Use libz to process ps.gz files

Process ps.gz files using GZlibDecompressor instead of spawing gunzip. Because tracker-extract runs the file parsers inside a seccomp sandbox, spawning an external process during parsing is not a good idea because it leaves us with little control on which syscalls are used.

Also added some functional tests for ps and ps.gz files.

Closes: #61 (closed)