Raise the accessibility tests execution time allowance

testMediaEventsTimelineScreen timed out on CI after 2 minutes. The spindump
shows the runner parked inside XCUIApplication.terminate() with the app
process alive but completely idle, so the audits had already finished and the
teardown alone exhausted the remaining budget.

The test is also the heaviest one in the plan, 120s on CI against 88s for the
next slowest and only 19s locally, since everything runs with the address and
undefined behaviour sanitizers enabled. Doubling the allowance gives all of
them enough headroom.

AccessibilityTests/SupportingFiles/AccessibilityTests.xctestplan

@@ -21,7 +21,7 @@
         }
       ]
     },
-    "defaultTestExecutionTimeAllowance" : 120,
+    "defaultTestExecutionTimeAllowance" : 240,
     "environmentVariableEntries" : [
 
     ],