Minor Changes to 3 Tests/Samples (#3451)

No source changes. Act on some items that have come up in recent discussions.
- Sample 33_Chart_create_line creates a stacked line chart. According to @MarkBaker, the stacking is done on the wrong variable, and, even were that not the case, stacking is unusual for line charts. Since this is our primary sample showing how to create a line chart, remove the stacking. Another sample, with a more appropriate choice of chart (33_Chart_create_bar_stacked), still shows how to create a stacked chart.
- The test for reading a styled cell from Html is flawed. It sets a date format for a string date/time, but the format is applied only to numeric data, so the format, although set correctly, is ineffective. Keep the test, but add some explanation in the assertion, and add some new more effective tests, also with explanations in the assertions.
- Wrong namespace used for Writer/Xlsx/ConditionalFillTest.

tests/PhpSpreadsheetTests/Writer/Xlsx/ConditionalFillTest.php

@@ -1,6 +1,6 @@
 <?php
 
-namespace PhpOffice\PhpSpreadsheetTests\Reader\Xlsx;
+namespace PhpOffice\PhpSpreadsheetTests\Writer\Xlsx;
 
 use PhpOffice\PhpSpreadsheet\Shared\File;
 use PhpOffice\PhpSpreadsheet\Spreadsheet;