Sometimes an error happens in kernel or in test that leaves
output files in corrupted or incorrect state. While runner or
resume will just move on to executing next test, when generating
results it could end up with no results.json

  Try processing outputs a little more persistently and use any
output file left there, even if only dmesg.txt. Also, when no
useful output files were present, instead of breaking out add
notrun.

  Inform about processing results for each test so a problem
could be spotted more easily.

v2: removed ')' from 'notrun\n)' (Kamil)
 using bool var, added more prints about errors (Ryszard)
v3: reused open_for_reading, removed bool var (Krzysztof)
 closing only positive fds[] in close_outputs(), checking
 file sizes also if all opens succeeded (Kamil)
v4: reverting to v2 and addresing review comments (Krzysztof)
 closing only already opened file, drop early return when empty
 output files as this changes run status (Kamil)
v5: reverting to returning fals...