Implement autoupdate for file_test. (#3043)

I've migrated the toolchain autoupdate scripts here, I just need a
little more time to do the explorer side (which I need to check
performance, that may require multithreading as we do in the current
script). However, this felt substantial enough to share and it means I
can handle autoupdate in more of the toolchain, including preparatory
work for autoupdate on multi-file tests.

Once explorer is done I'll remove the old script.

toolchain/driver/driver_file_test.cpp

@@ -17,6 +17,13 @@ class DriverFileTest : public DriverFileTestBase {
   auto GetDefaultArgs() -> llvm::SmallVector<std::string> override {
     CARBON_FATAL() << "ARGS is always set in these tests";
   }
+
+  auto DoExtraCheckReplacements(std::string& check_line) -> void override {
+    // TODO: Disable token output, it's not interesting for these tests.
+    if (llvm::StringRef(check_line).starts_with("// CHECK:STDOUT: {")) {
+      check_line = "// CHECK:STDOUT: {{.*}}";
+    }
+  }
 };
 
 }  // namespace