kunit: string-stream: Add kunit_alloc_string_stream()

Add function kunit_alloc_string_stream() to do a resource-managed
allocation of a string stream, and corresponding
kunit_free_string_stream() to free the resource-managed stream.

This is preparing for decoupling the string_stream
implementation from struct kunit, to reduce the amount of code
churn when that happens. Currently:
 - kunit_alloc_string_stream() only calls alloc_string_stream().
 - kunit_free_string_stream() takes a struct kunit* which
   isn't used yet.

Callers of the old alloc_string_stream() and
string_stream_destroy() are all requesting a managed allocation
so have been changed to use the new functions.

alloc_string_stream() has been temporarily made static because
its current behavior has been replaced with
kunit_alloc_string_stream().

Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>