Merge branch 'jk/test-shell-trace'

Test scripts were taught to notice "-x" option to show shell trace, as if the tests were run under "sh -x". * jk/test-shell-trace: test-lib.sh: support -x option for shell-tracing t5304: use helper to report failure of "test foo = bar" t5304: use test_path_is_* instead of "test -f"