fix run_test.m on Windows

On windows, testing needs to strip \r, since the *.exp files are eol=\n and Mercury output is
using the native line ending by default (c.f. io.(write|print)_line/3)
Nevertheless, not all tests pass, but before, all test aborted since the exit code was 1.
There is another potential bug, it should have hit the "FAILED" disjunction, but maybe io.call_system/3 works this way.

tests/test_json.m:
    on line 121, added --strip-trailing-cr parameter (which is available in msys diff)