ktest: Allow options to be used by other options

There are cases where one ktest option may be used within another
ktest option. Allow them to be reused just like config variables
but there are evaluated at time of test not config processing time.

Thus having something like:

MAKE_CMD = make ARCH=${ARCH}

TEST_START
ARCH = powerpc

TEST_START
ARCH = arm

Will have the arch defined for each test iteration.

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