Replace usages of assertions with strstr with assertStringContainsString()/assertStringNotContainsString()

Follow up from #3128746: Replace assertions involving calls to strpos() with more accurate string assertions. There are a number of cases where we are using strstr() or stristr() to assert whether a string contains another string. We should replace these with assertStringContainsString()/assertStringNotContainsString() instead.

Found 8 occurrences:

grep -nri '\->assert.*stri\?str' core
core/modules/layout_builder/tests/src/FunctionalJavascript/BlockFormMessagesTest.php:108:    $this->assertTrue(stristr($top_form_elements[0]->getText(), 'Title field is required.') !== FALSE);
core/modules/image/tests/src/Functional/ImageFieldDisplayTest.php:153:      $this->assertTrue(strstr($this->drupalGetHeader('Cache-Control'), 'private') !== FALSE, 'Cache-Control header was sent.');
core/modules/system/tests/src/FunctionalJavascript/OffCanvasTest.php:61:        $this->assertTrue(strstr($style, 'width: 555px;') !== FALSE, 'Dialog width respected.');
core/modules/system/tests/src/FunctionalJavascript/OffCanvasTest.php:65:        $this->assertTrue(strstr($style, 'width: 555px;') === FALSE, 'Dialog width reset to default.');
core/modules/system/tests/src/FunctionalJavascript/OffCanvasTest.php:80:        $this->assertTrue((bool) strstr($style, 'height: auto;'));
core/modules/system/tests/src/FunctionalJavascript/OffCanvasTest.php:83:        $this->assertTrue((bool) strstr($style, 'height: 421px;'));
core/modules/settings_tray/tests/src/FunctionalJavascript/QuickEditIntegrationTest.php:154:    $this->assertTrue(strstr($href, "/admin/structure/block/manage/custom/settings-tray?destination=$destination") !== FALSE);
core/modules/responsive_image/tests/src/Functional/ResponsiveImageFieldDisplayTest.php:265:      $this->assertTrue(strstr($this->drupalGetHeader('Cache-Control'), 'private') !== FALSE, 'Cache-Control header was sent.');