[gtest] Fixed a rogue std::isalnum

Use IsAlNum instead (gtest-port.h), which deals with char signedness correctly. This was the only spot in gtest where a cctype function was called instead of its gtest-port.h equivalent.

Addresses https://github.com/assimp/assimp/issues/3867 and then some.

contrib/gtest/include/gtest/internal/gtest-param-util.h

@@ -644,7 +644,7 @@ class ParameterizedTestCaseInfo : public ParameterizedTestCaseInfoBase {
 
     // Check for invalid characters
     for (std::string::size_type index = 0; index < name.size(); ++index) {
-      if (!isalnum(name[index]) && name[index] != '_')
+      if (!IsAlNum(name[index]) && name[index] != '_')
         return false;
     }