Add a test that triggers the problem to ensure it stays fixed.

Conflicts:
	tests/test_util_file.c

The test is implied in test1.

The idea is to check that the returned lengths and strings
are identical to what is expected to return the already
tested function 'json_object_to_json_string_ext'.

Signed-off-by: José Bollo <jose.bollo@iot.bzh>

Change %d to %llu and add cast to unsigned long long for size_t arguments,
otherwise compilation will fail with errors like:

test1.c:70:15: error: format ‘%d’ expects argument of type ‘int’, but argument 2 has type ‘size_t {aka long unsigned int}’ [-Werror=format=]

%zu is avoided to stay compatible with old libc versions (like old Visual
Studio).

Tests added.

Extend the run_output_test() function so we actually can pass command line
 parameters and so we can support different output files for the same test
 executable.
Also provide some hints about what to do if a test fails (i.e. set VERBOSE=1).