[lldb][test] Add test for printing std::string through expression evaluator

This would've caught the failures in
llvm#105865 in the libc++
data-formatter CI.

Author: Michael137

lldb/test/API/functionalities/data-formatter/data-formatter-stl/libcxx/string/TestDataFormatterLibcxxString.py

@@ -100,6 +100,16 @@ def cleanup():
             "s", result_type=ns + "::wstring", result_summary='L"hello world! מזל טוב!"'
         )
 
+        self.expect_expr(
+            "q", result_type=ns + "::string", result_summary='"hello world"'
+        )
+
+        self.expect_expr(
+            "Q",
+            result_type=ns + "::string",
+            result_summary='"quite a long std::strin with lots of info inside it"',
+        )
+
         self.expect(
             "frame variable",
             substrs=[