taskcluster: raise testharness chunks to 16 (#19724)

* Some testharness runs are timing out on the daily runs of webkitgtk.
* Before trying to raise the timeout, try to raise the number of chunks.

Author: clopez

.taskcluster.yml

@@ -25,21 +25,22 @@ tasks:
               }
           each(browser):
             $map:
-              - [testharness, 1, 15]
-              - [testharness, 2, 15]
-              - [testharness, 3, 15]
-              - [testharness, 4, 15]
-              - [testharness, 5, 15]
-              - [testharness, 6, 15]
-              - [testharness, 7, 15]
-              - [testharness, 8, 15]
-              - [testharness, 9, 15]
-              - [testharness, 10, 15]
-              - [testharness, 11, 15]
-              - [testharness, 12, 15]
-              - [testharness, 13, 15]
-              - [testharness, 14, 15]
-              - [testharness, 15, 15]
+              - [testharness, 1, 16]
+              - [testharness, 2, 16]
+              - [testharness, 3, 16]
+              - [testharness, 4, 16]
+              - [testharness, 5, 16]
+              - [testharness, 6, 16]
+              - [testharness, 7, 16]
+              - [testharness, 8, 16]
+              - [testharness, 9, 16]
+              - [testharness, 10, 16]
+              - [testharness, 11, 16]
+              - [testharness, 12, 16]
+              - [testharness, 13, 16]
+              - [testharness, 14, 16]
+              - [testharness, 15, 16]
+              - [testharness, 16, 16]
               - [reftest, 1, 5]
               - [reftest, 2, 5]
               - [reftest, 3, 5]