kunit: string-stream-test: Add cases for string_stream newline appending

Add test cases for testing the string_stream feature that appends a
newline to strings that do not already end with a newline.

string_stream_no_auto_newline_test() tests with this feature disabled.
Newlines should not be added or dropped.

string_stream_auto_newline_test() tests with this feature enabled.
Newlines should be added to lines that do not end with a newline.

string_stream_append_auto_newline_test() tests appending the
content of one stream to another stream when the target stream
has newline appending enabled.

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