kunit: Add parent kunit for parameterized test context

Currently, KUnit parameterized tests lack a mechanism to share
resources across parameter runs because the same `struct kunit`
instance is cleaned up and reused for each run.

This patch introduces parameterized test context, enabling test
users to share resources between parameter runs. It also allows
setting up resources that need to be available for all parameter
runs only once, which is helpful in cases where setup is expensive.

To establish a parameterized test context, this patch adds a
parent pointer field to `struct kunit`. This allows resources added
to the parent `struct kunit` to be shared and accessible across all
parameter runs.

In kunit_run_tests(), the default `struct kunit` created is now
designated to act as the parameterized test context whenever a test
is parameterized.

Subsequently, a new `struct kunit` is made for each parameter run, and
its parent pointer is set to the `struct kunit` that holds the
parameterized test context.

Link: https://lore.kernel.org/r/20250826091341.1427123-2-davidgow@google.com


Reviewed-by: David Gow <davidgow@google.com>
Reviewed-by: Rae Moar <rmoar@google.com>
Signed-off-by: Marie Zhussupova <marievic@google.com>
Signed-off-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>