Fix cpp/redos test: swap non-exponential (a|ab)* for (a*)* and populate expected

Author: Copilot

cpp/ql/test/query-tests/Security/CWE/CWE-1333-ReDoS/ReDoS.expected

@@ -0,0 +1,3 @@
+| test.cpp:58:26:58:27 | a+ | This part of the regular expression may cause exponential backtracking on strings containing many repetitions of 'a'. |
+| test.cpp:64:25:64:30 | (a\|a)* | This part of the regular expression may cause exponential backtracking on strings containing many repetitions of 'a'. |
+| test.cpp:70:26:70:27 | a* | This part of the regular expression may cause exponential backtracking on strings containing many repetitions of 'a'. |

cpp/ql/test/query-tests/Security/CWE/CWE-1333-ReDoS/test.cpp

@@ -67,7 +67,7 @@ void test_exp_redos(const std::string& input) {
 
     // BAD: alternation with overlapping branches inside a repetition.
     {
-        std::regex re("^(a|ab)*$");
+        std::regex re("^(a*)*$");
         std::regex_match(input, re);
     }