test: fix common.canCreateSymLink() on non-Windows

test/common/README.md indicates that canCreateSymlink() always returns
true on non-Windows platforms. However, prior to this commit, undefined
was being returned. This commit aligns the implementation with the
docs by returning true.

PR-URL: #20511
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

test/common/index.js

@@ -518,6 +518,8 @@ exports.canCreateSymLink = function() {
       return false;
     }
   }
+  // On non-Windows platforms, this always returns `true`
+  return true;
 };
 
 exports.getCallSite = function getCallSite(top) {