lib/igt_kmod: add compatibility for KUnit

This adds functions for both executing the tests as well as parsing (K)TAP
kmsg output, as per the KTAP spec [1].

[1] https://www.kernel.org/doc/html/latest/dev-tools/ktap.html



v1 -> v2:
- refactor igt_kunit function and ktap parser so that we have only one
  parser that we call only once (code size is now less than half the
  size as v1)
- add lookup_value helper
- fix parsing problems
v2 -> v3:
- move ktap parsing functions to own file
- rename to ktap_parser
- get rid of unneeded pointers in igt_kunit
- change return values to allow for subsequent call to igt_kselftests if
  needed
- add docs to parsing functions and helpers
- switch to line buffering
- add line buffering logging helper
- fix kunit module handling
- fix parsing of version lines

Signed-off-by: Isabella Basso <isabbasso@riseup.net>