kunit: string-stream: Decouple string_stream from kunit

Re-work string_stream so that it is not tied to a struct kunit. This is
to allow using it for the log of struct kunit_suite.

Instead of resource-managing individual allocations the whole string_stream
can be resource-managed, if required.

    alloc_string_stream() now allocates a string stream that is
    not resource-managed.

    string_stream_destroy() now works on an unmanaged string_stream
    allocated by alloc_string_stream() and frees the entire
    string_stream (previously it only freed the fragments).

    string_stream_clear() has been made public for callers that
    want to free the fragments without destroying the string_stream.

For resource-managed allocations use kunit_alloc_string_stream()
and kunit_free_string_stream().

In addition to this, string_stream_get_string() now returns an
unmanaged buffer that the caller must kfree().

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