[render-test] Fix regex matching the directory name

This code was matching `style.json` instead of `metrics.json` if
the `style.json` was contained inside `metrics/.../style.json`.

render-test/parser.cpp

@@ -289,17 +289,17 @@ std::vector<std::string> readExpectedEntries(const std::regex& regex, const mbgl
 } // namespace
 
 std::vector<std::string> readExpectedImageEntries(const mbgl::filesystem::path& base) {
-    static const std::regex regex(".*expected.*.png");
+    static const std::regex regex(".*/expected.*.png");
     return readExpectedEntries(regex, base);
 }
 
 std::vector<std::string> readExpectedMetricEntries(const mbgl::filesystem::path& base) {
-    static const std::regex regex(".*metrics.*.json");
+    static const std::regex regex(".*/metrics.*.json");
     return readExpectedEntries(regex, base);
 }
 
 std::vector<std::string> readExpectedJSONEntries(const mbgl::filesystem::path& base) {
-    static const std::regex regex(".*expected.*.json");
+    static const std::regex regex(".*/expected.*.json");
     return readExpectedEntries(regex, base);
 }