kunit: string-stream: Improve testing of string_stream

Replace the minimal tests with more-thorough testing.

string_stream_init_test() tests that struct string_stream is
initialized correctly.

string_stream_line_add_test() adds a series of numbered lines and
checks that the resulting string contains all the lines.

string_stream_variable_length_line_test() adds a large number of
lines of varying length to create many fragments, then tests that all
lines are present.

string_stream_append_test() tests various cases of using
string_stream_append() to append the content of one stream to another.

Adds string_stream_append_empty_string_test() to test that adding an
empty string to a string_stream doesn't create a new empty fragment.

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>