Tests: Refactor var naming
Tests: Refactor var naming

Replace `fin` with `exit_code` as discussed in [1].

[1]
https://github.com/coreos/tectonic-installer/pull/2436/files#r152238782

metral/tectonic-installer,kyoto/tectonic-installer,mrwacky42/tectonic-installer,zbwright/tectonic-installer,justaugustus/tectonic-installer,alexsomesan/tectonic-installer,hhoover/tectonic-installer,s-urbaniak/tectonic-installer,kalmog/tectonic-installer,hhoover/tectonic-installer,kyoto/tectonic-installer,kalmog/tectonic-installer,s-urbaniak/tectonic-installer,squat/tectonic-installer,zbwright/tectonic-installer,alexsomesan/tectonic-installer,lander2k2/tectonic-installer,squat/tectonic-installer,rithujohn191/tectonic-installer,metral/tectonic-installer,cpanato/tectonic-installer,rithujohn191/tectonic-installer,s-urbaniak/tectonic-installer,justaugustus/tectonic-installer,kyoto/tectonic-installer,rithujohn191/tectonic-installer,hhoover/tectonic-installer,cpanato/tectonic-installer,lander2k2/tectonic-installer,alexsomesan/tectonic-installer,s-urbaniak/tectonic-installer,mrwacky42/tectonic-installer,alexsomesan/tectonic-installer,mrwacky42/tectonic-installer,rithujohn191/tectonic-installer,zbwright/tectonic-installer,metral/tectonic-installer,alexsomesan/tectonic-installer,metral/tectonic-installer,lander2k2/tectonic-installer,justaugustus/tectonic-installer,lander2k2/tectonic-installer,squat/tectonic-installer,hhoover/tectonic-installer,cpanato/tectonic-installer,derekhiggins/installer,justaugustus/tectonic-installer,cpanato/tectonic-installer,zbwright/tectonic-installer,justaugustus/tectonic-installer,s-urbaniak/tectonic-installer,mrwacky42/tectonic-installer,derekhiggins/installer,mrwacky42/tectonic-installer,kalmog/tectonic-installer,zbwright/tectonic-installer,kyoto/tectonic-installer,metral/tectonic-installer,squat/tectonic-installer,rithujohn191/tectonic-installer,hhoover/tectonic-installer,kyoto/tectonic-installer