ktest.pl: Add -D option to override options

Add -D option that lets the user override options in the config.

For instance, if the config has: BUILD_NOCLEAN=1 which prevents mrproper
from being called before builds, and the user wants to call it once. The
user can run:

  ktest -D BUILD_NOCLEAN=0 config

And the default "BUILD_NOCLEAN" options will be disabled.

If the user wants to change the second test to do a build and not boot,
the user can run:

  ktest -D 'TEST_TYPE[2]=build' config

Where the '[#]' is for the test to assign the variable for. In the above
example, it will happen on test 2.

Cc: "John Warthog9 Hawley" <warthog9@kernel.org>
Cc: Dhaval Giani <dhaval.giani@gmail.com>
Cc: Greg KH <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/20250718202053.231478909@kernel.org


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