runner: Fix use of newline on arguments

Currently there's no support for newlines on arguments passed to runner.
However it's also a silent failure:

	# igt_runner --test-list '/tmp/test
	list2.txt' build/tests/ /tmp/results

	# head /tmp/results/metadata.txt
	disk_usage_limit : 0
	test_list : /tmp/test
	list2.txt
	name : results
	...

	# ./build/runner/igt_resume /tmp/results
	[9840425.334900] All tests already executed.
	resume failed at generating results
	Done.

Embedding a newline like this is very dubious for test-list, but it's
used for e.g. hooks. In future we will add the command line to the
metadata and possibly migrate the hooks, so add support for
escaping/unescaping the string on save/restore.

The method chosen is slightly different than the one used for hooks:
instead of adding a escape char and keeping the char escaped, this just
prefers using an hex representation of the char with a \x<HEX>h
sequence. This makes it easier when unescaping since the reader can
continue reading one line per iteration. In future this can also be
adopted by the hooks or even migrating the hooks to use metadata.txt.

Another fix is that now we just skip null values on the serialization
side.  Previously it would serialize "(null)" and then load that string
instead of NULL. Add code_coverage_script to the runner_test to cover
that, which would previously fail.

Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Tested-by: Peter Senna Tschudin <peter.senna@linux.intel.com>
Reviewed-by: Peter Senna Tschudin <peter.senna@linux.intel.com>
Link: https://lore.kernel.org/r/20250207231039.2883195-9-lucas.demarchi@intel.com


Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>