ktest: Allow a test option to use its default option

Options are allowed to use other options, for example:

  LOG_FILE = ${OUTPUT_DIR}/${MACHINE}.log

where the option LOG_FILE used the options OUTPUT_DIR and MACHINE.

But if a test option were to use a default option, it will not get
substituted:

  OUTPUT_DIR = ${THIS_DIR}/${MACHINE}

  TEST_START
  OUTPUT_DIR = ${OUTPUT_DIR}/t1

For the above test, OUTPUT_DIR will stay literally "${OUTPUT_DIR}/t1"
and not be converted to "${THIS_DIR}/${MACHINE}/t1". When the test runs,
it will pass the ${OUTPUT_DIR} to the shell, which would probaly
interpret it as "", and the output directory will end up as "/t1".

Change the code where if a test option has its own option name in
its defined field, and a default option exists, then substitute the
default option in its place.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>