ktest updates for v6.9:

- Allow variables to contain variables. This makes the shell commands
  have a bit more flexibility to reuse existing variables.

- Have make_warnings_file in build-only mode require limited variables

  The make_warnings_file test will create a file with all existing
  warnings (which can be used to compare against in builds with
  new commits). Add it to the build-only list that doesn't require
  other variables (like how to reset a machine), as the make_warnings_file
  makes the most sense on build only tests.
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