Fix test reference for SymbolTable output

clang-8 may write '(nil)' instead of '0' in output

tests/test_SymbolTable.cpp

@@ -24,11 +24,13 @@ TEST_CASE("SymbolTable", "[language]")
       std::stringstream st_output;
       st_output << '\n' << *root_st;
 
-      REQUIRE(st_output.str() == R"(
--- Symbol table state -- parent : 0
- a: --
-------------------------
-)");
+      std::stringstream expected_output;
+      expected_output << '\n'
+                      << "-- Symbol table state -- parent : " << static_cast<SymbolTable*>(nullptr) << '\n'
+                      << " a: --\n"
+                      << "------------------------\n";
+
+      REQUIRE(st_output.str() == expected_output.str());
     }
 
     SECTION("Attributes")
@@ -68,11 +70,13 @@ TEST_CASE("SymbolTable", "[language]")
         std::stringstream st_output;
         st_output << '\n' << *root_st;
 
-        REQUIRE(st_output.str() == R"(
--- Symbol table state -- parent : 0
- a: 2.3
-------------------------
-)");
+        std::stringstream expected_output;
+        expected_output << '\n'
+                        << "-- Symbol table state -- parent : " << static_cast<SymbolTable*>(nullptr) << '\n'
+                        << " a: 2.3\n"
+                        << "------------------------\n";
+
+        REQUIRE(st_output.str() == expected_output.str());
       }
     }
   }