test-container: add exec, cwd

exec <path_to_test_binary> [optional_argv_0]

  copies test binary to specified location and runs it from
  there.  If the second argument is provided, that will
  be used for argv[0]

cwd <directory>

  attempts to chdir(directory) before running test

Note: "cwd" not "cd" as it takes effect just before the
test binary runs, not when it's encountered in the script,
so it can't be used as a path shortcut like "cd" would imply.

cleanup: use xstrdup() instead of strdup()

Reviewed-by: Carlos O'Donell <carlos@redhat.com>