Merge branch 'jk/test-shell-trace'

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"