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.

toolchain/testing/file_test.cpp

@@ -15,7 +15,6 @@
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Support/FormatVariadic.h"
 #include "llvm/Support/VirtualFileSystem.h"
-#include "testing/file_test/file_system.h"
 #include "testing/file_test/file_test_base.h"
 #include "toolchain/driver/driver.h"
 
@@ -98,6 +97,21 @@ auto ToolchainFileTest::GetArgReplacements() const
   return {{"core_package_dir", installation_.core_package()}};
 }
 
+// Adds a file to the fs.
+static auto AddFile(llvm::vfs::InMemoryFileSystem& fs, llvm::StringRef path)
+    -> ErrorOr<Success> {
+  llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> file =
+      llvm::MemoryBuffer::getFile(path);
+  if (file.getError()) {
+    return ErrorBuilder() << "Getting `" << path
+                          << "`: " << file.getError().message();
+  }
+  if (!fs.addFile(path, /*ModificationTime=*/0, std::move(*file))) {
+    return ErrorBuilder() << "Duplicate file: `" << path << "`";
+  }
+  return Success();
+}
+
 auto ToolchainFileTest::Run(
     const llvm::SmallVector<llvm::StringRef>& test_args,
     llvm::IntrusiveRefCntPtr<llvm::vfs::InMemoryFileSystem>& fs,