WTB to BTB, add getRawContent() in BTB

Problem/Motivation

All the following WTB tests use getRawContent() which doesn't exist in BTB.

AddFeedTest
BigPipeTest
BlockRenderOrderTest
CommentBlockTest
CommentPreviewTest
CommentTestBase
CommentNonNodeTest
CommentTestBase
CommentRestExportTest
CommentUserNameTest
ConfigExportImportUITest
ConfigExportUITest
EditorAdminTest
EditorLoadingTest
ManageDisplayTest
FileListingTest
NumberFieldRdfaTest
StyleSerializerTest
FormTest
ThemeTest
RenderWebTest
SelectPagerDefaultTest
SelectTableSortDefaultTest
TemporaryQueryTest
LanguageSelectElementTest
DestinationTest
SessionAuthenticationTest
SessionTest
ShutdownFunctionsTest
EngineTwigTest
ThemeSuggestionsAlterTest
UpdateContribTest
UserBlocksTest
ViewsFormTest
ItemsPerPageTest
SortingTest
PreviewTest

Proposed resolution

Create getRawContent() in BTB so we can easily convert WTB to BTB as defined in #2735005: Convert all Simpletest web tests to BrowserTestBase (or UnitTestBase/KernelTestBase)