Switch FileTest to use StringRefs instead of files. (#2885)

In explorer, we already support parsing a string_view, so use that. In toolchain, we need to build support, probably using vfs, so that's a todo.

bazel test //explorer:file_test --runs_per_test=5

- branch: Stats over 250 runs: max = 18.3s, min = 5.2s, avg = 11.2s, dev = 2.9s
- trunk: Stats over 250 runs: max = 22.3s, min = 5.9s, avg = 12.1s, dev = 2.8s

Not a dramatic improvement, but maybe more effective long-term, and this'd been requested on #2876

explorer/file_test.cpp

@@ -32,7 +32,7 @@ class ParseAndExecuteTestFile : public FileTestBase {
     }
   }
 
-  auto RunWithFiles(const llvm::SmallVector<std::string>& test_files,
+  auto RunWithFiles(const llvm::SmallVector<TestFile>& test_files,
                     llvm::raw_ostream& stdout, llvm::raw_ostream& stderr)
       -> bool override {
     if (test_files.size() != 1) {
@@ -58,9 +58,9 @@ class ParseAndExecuteTestFile : public FileTestBase {
 
     // Run the parse. Parser debug output is always off because it's difficult
     // to redirect.
-    auto result =
-        ParseAndExecuteFile(prelude_path, path().filename().string(),
-                            /*parser_debug=*/false, &trace_stream, &stdout);
+    auto result = ParseAndExecute(
+        prelude_path, test_files[0].filename, test_files[0].content,
+        /*parser_debug=*/false, &trace_stream, &stdout);
     // This mirrors printing currently done by main.cpp.
     if (result.ok()) {
       stdout << "result: " << *result << "\n";