autoupdate: mention in the output tests that would still fail after the update (#7877)

If a test would still fail post-autoupdate due to either mismatched
`fail_` prefix or due to being NOAUTOUPDATE, the test is reported as `?`
instead of `.` in the incremental progress output, and a message is
listed after the progress output describing the problem. For example:

```console
Problems that require manual fixes:
  - `toolchain/check/testdata/basics/zz_prefix_probe.carbon` split `bad_missing_prefix.carbon` failed; if failure is expected, add the `fail_` prefix.
  - `toolchain/driver/testdata/fail_config.carbon` succeeded; if success is expected, remove the `fail_` prefix.
```

This change caused the `NOAUTOUPDATE` test
`toolchain/driver/testdata/fail_config.carbon` to fail, because when run
under autoupdate (or with `bazel run`), the install manifest would be
present, and the test is checking that it's not. Fixed by using the VFS
to look for the manifest, which then required fixing
toolchain/driver:driver_test to set up a real filesystem VFS so that it
*could* find the manifest.

With this change, there should no longer be a reason to manually run
file_test in addition to running autoupdate.

Assisted-by: Claude via Antigravity

testing/file_test/test_file.cpp

@@ -893,8 +893,10 @@ static auto ProcessFileContent(llvm::StringRef filename,
   return Success();
 }
 
-auto ProcessTestFile(llvm::StringRef test_name, bool running_autoupdate)
-    -> ErrorOr<TestFile> {
+// Implementation of `ProcessTestFile`, without special handling for
+// NOAUTOUPDATE files during autoupdate.
+static auto ProcessTestFileImpl(llvm::StringRef test_name,
+                                bool running_autoupdate) -> ErrorOr<TestFile> {
   TestFile test_file;
 
   // Store the original content, to avoid a read when autoupdating.
@@ -976,4 +978,16 @@ auto ProcessTestFile(llvm::StringRef test_name, bool running_autoupdate)
   return std::move(test_file);
 }
 
+auto ProcessTestFile(llvm::StringRef test_name, bool running_autoupdate)
+    -> ErrorOr<TestFile> {
+  auto result = ProcessTestFileImpl(test_name, running_autoupdate);
+
+  // Autoupdate won't modify a NOAUTOUPDATE file, so process it as normal. This
+  // builds the expectations, which lets us report mismatches to the user.
+  if (running_autoupdate && result.ok() && !result->autoupdate_line_number) {
+    return ProcessTestFileImpl(test_name, /*running_autoupdate=*/false);
+  }
+  return result;
+}
+
 }  // namespace Carbon::Testing