Disable Clang 18 C++26 libstdc++ unit tests due to incompatibility with version 15

See bemanproject/exemplar@48638ee

Author: ednolan

.github/workflows/ci_tests.yml

@@ -93,14 +93,28 @@ jobs:
                 }
               ]
             },
-            { "versions": ["20", "19", "18"],
+            { "versions": ["20", "19"],
               "tests": [
                 { "cxxversions": ["c++26", "c++23", "c++20"],
                   "tests": [
                     {"stdlibs": ["libstdc++", "libc++"], "tests": ["Release.Default"]}
                   ]
                 }
               ]
+            },
+            { "versions": ["18"],
+              "tests": [
+                { "cxxversions": ["c++26", "c++23", "c++20"],
+                  "tests": [
+                    {"stdlibs": ["libc++"], "tests": ["Release.Default"]}
+                  ]
+                },
+                { "cxxversions": ["c++23", "c++20"],
+                  "tests": [
+                    {"stdlibs": ["libc++"], "tests": ["Release.Default"]}
+                  ]
+                }
+              ]
             }
           ],
           "appleclang": [