runner: Exit 8 if every test was skipped or otherwise ignored

Simon McVittierequested to merge
smcv/autopkgtest:exit-8 into master

Closes: #901804

Probably shouldn't be merged until https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=901846 is implemented in debci and https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=901847 in britney.

Edited by Simon McVittie

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