Make INCLUDE-FILE less sensitive to path changes (#5159)

It's helpful for stability to not have the path in the repo reflected in
test files, something I'm separately running into. So to reduce this,
align INCLUDE-FILE with other split behavior:

- Use the filename (with a "include_files/" subdir to disambiguate),
rather than the full path.
- Note if we eventually want to support splits in these, the same
approach could be extended.
- Only provide as an arg if the user requests files as args.

Factoring AddFile back because it's hard to share; I'm also advocating
to remove the prelude manifest, which would mean the remaining call
could be removed.

testing/file_test/test_file.cpp

@@ -567,14 +567,18 @@ static auto TryConsumeArgs(llvm::StringRef line, llvm::StringRef line_trimmed,
   return true;
 }
 
-static auto TryConsumeIncludeFile(llvm::StringRef line_trimmed,
-                                  llvm::SmallVector<std::string>* include_files)
-    -> ErrorOr<bool> {
+static auto TryConsumeIncludeFile(
+    llvm::StringRef line_trimmed,
+    llvm::SmallVector<TestFile::Split>* include_files) -> ErrorOr<bool> {
   if (!line_trimmed.consume_front("// INCLUDE-FILE: ")) {
     return false;
   }
 
-  include_files->push_back(std::string(line_trimmed));
+  std::filesystem::path path = std::string(line_trimmed);
+  CARBON_ASSIGN_OR_RETURN(std::string content, ReadFile(path));
+  include_files->push_back(
+      {.filename = std::filesystem::path("include_files") / path.filename(),
+       .content = content});
   return true;
 }
 
@@ -686,7 +690,7 @@ auto ProcessTestFile(llvm::StringRef test_name, bool running_autoupdate)
     }
     CARBON_ASSIGN_OR_RETURN(
         is_consumed,
-        TryConsumeIncludeFile(line_trimmed, &test_file.include_files));
+        TryConsumeIncludeFile(line_trimmed, &test_file.include_file_splits));
     if (is_consumed) {
       continue;
     }