Refactor FileTest construction so that the test class is directly available. (#3035)

This is a simplification of the construction, although somewhat limiting
(it means that the caller can't register the same file multiple times,
though I stopped doing that anyways since it was causing confusion).
What this more importantly _allows_ is logic on the FileTestBase child
itself that's not test-specific -- in particular, autoupdate
functionality which wouldn't use RUN_ALL_TESTS.

---------

Co-authored-by: Chandler Carruth <chandlerc@gmail.com>

toolchain/codegen/codegen_file_test.cpp

@@ -2,11 +2,9 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
-#include <filesystem>
 #include <string>
 
 #include "llvm/ADT/SmallVector.h"
-#include "llvm/ADT/StringRef.h"
 #include "toolchain/driver/driver_file_test_base.h"
 
 namespace Carbon::Testing {
@@ -23,9 +21,6 @@ class CodeGenFileTest : public DriverFileTestBase {
 
 }  // namespace
 
-auto RegisterFileTests(const llvm::SmallVector<std::filesystem::path>& paths)
-    -> void {
-  CodeGenFileTest::RegisterTests<CodeGenFileTest>("CodeGenFileTest", paths);
-}
+CARBON_FILE_TEST_FACTORY(CodeGenFileTest);
 
 }  // namespace Carbon::Testing