Fix a path problem in onnxruntime_perf_test (#21341)

### Description
Resolve #21267 . onnxruntime_perf_test does not work properly if the
input model path url is just a single filename without any path
separator. For example,

```
./onnxruntime_perf_test -t 10 model.onnx
```

The problem was introduced in #19196 by me.

onnxruntime/test/onnx/TestCase.h

@@ -53,7 +53,8 @@ class TestModelInfo {
  public:
   virtual const std::filesystem::path& GetModelUrl() const = 0;
   virtual std::filesystem::path GetDir() const {
-    return GetModelUrl().parent_path();
+    const auto& p = GetModelUrl();
+    return p.has_parent_path() ? p.parent_path() : std::filesystem::current_path();
   }
   virtual const std::string& GetNodeName() const = 0;
   virtual const ONNX_NAMESPACE::ValueInfoProto* GetInputInfoFromModel(size_t i) const = 0;