Add file_test support for specifying arguments and only checking a subset of output. (#3018)

This is sufficient to convert remaining toolchain tests to file_test. %s
and %t are currently used. While #2978 might use %T with lit, I think
that's resolving a lit-specific issue that's not necessary when
TEST_TMPDIR is readily available for any necessary operations (i.e.,
setting the working directory). With ARGS, I think it's feasible to
switch to file_test without %T.

Also adds documentation to file_test_base.h, which was starting to feel
like a significant gap.

This is not yet handled by autoupdate, but I'm eyeing that next.

toolchain/codegen/codegen_file_test.cpp

@@ -0,0 +1,31 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// 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 {
+namespace {
+
+class CodeGenFileTest : public DriverFileTestBase {
+ public:
+  using DriverFileTestBase::DriverFileTestBase;
+
+  auto GetDefaultArgs() -> llvm::SmallVector<std::string> override {
+    CARBON_FATAL() << "ARGS is always set in these tests";
+  }
+};
+
+}  // namespace
+
+auto RegisterFileTests(const llvm::SmallVector<std::filesystem::path>& paths)
+    -> void {
+  CodeGenFileTest::RegisterTests<CodeGenFileTest>("CodeGenFileTest", paths);
+}
+
+}  // namespace Carbon::Testing