runner/json_tests: Test handling of unprintable output from tests

A simple test output with numbers from 1 to 255, both in plain text
form and as a single byte with that particular value.

Note that the json spec doesn't require \u-encoding for characters
other than '"', '\' and the range U+0000 to U+001F, the raw
non-\u-encoded UTF-8 in the reference.json file for bytes 128 and up
is what libjson-c outputs for those codepoints and is valid.

The validity of the json file can be verified with iconv, i.e.

 $ iconv -f UTF-8 reference.json -o /dev/null && echo it is utf-8

v2: Rebase over dynamic subtest tests, trivial

Signed-off-by: Petri Latvala <petri.latvala@intel.com>
Cc: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Reviewed-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>