kunit: string-stream: Test performance of string_stream

Add a test of the speed and memory use of string_stream.

string_stream_performance_test() doesn't actually "test" anything (it
cannot fail unless the system has run out of allocatable memory) but it
measures the speed and memory consumption of the string_stream and reports
the result.

This allows changes in the string_stream implementation to be compared.

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