test: change isAix to isAIX

This makes the naming more consistent with existing properties like
isFreeBSD where the capitalization of the property name is consistent
with the conventional styling of the operating system.

PR-URL: https://github.com/nodejs/node/pull/14263
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Bryan English <bryan@bryanenglish.com>

test/tick-processor/test-tick-processor-cpp-core.js

@@ -6,7 +6,7 @@ if (!common.enoughTestCpu)
 
 if (common.isWindows ||
     common.isSunOS ||
-    common.isAix ||
+    common.isAIX ||
     common.isLinuxPPCBE ||
     common.isFreeBSD)
   common.skip('C++ symbols are not mapped for this os.');