Issue #2802923 by Wim Leers, casey: Use <span> instead of <div> for BigPipe placeholders

core/modules/big_pipe/src/Render/BigPipe.php

...dules/big_pipe/src/Render/Placeholder/BigPipeStrategy.php

...odules/big_pipe/src/Tests/BigPipePlaceholderTestCases.php

core/modules/big_pipe/src/Tests/BigPipeTest.php

...pipe_regression_test/big_pipe_regression_test.routing.yml

...e_regression_test/src/BigPipeRegressionTestController.php

.../tests/src/FunctionalJavascript/BigPipeRegressionTest.php