ktest.pl: Prevent recursion of default variable options

If a default variable contains itself, do not recurse on it.

For example:

  ADD_CONFIG := ${CONFIG_DIR}/temp_config
  DEFAULTS
  ADD_CONFIG = ${CONFIG_DIR}/default_config ${ADD_CONFIG}

The above works because the temp variable ADD_CONFIG (is a temp because it
is created with ":=") is already defined, it will be substituted in the
variable option. But if it gets commented out:

  # ADD_CONFIG := ${CONFIG_DIR}/temp_config
  DEFAULTS
  ADD_CONFIG = ${CONFIG_DIR}/default_config ${ADD_CONFIG}

Then the above will go into a recursive loop where ${ADD_CONFIG} will
get replaced with the current definition of ADD_CONFIG which contains the
${ADD_CONFIG} and that will also try to get converted. ktest.pl will error
after 100 attempts of recursion and fail.

When replacing a variable with the default variable, if the default
variable contains itself, do not replace it.

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/20250718202053.732189428@kernel.org
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

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