tests: Prevent a rare-ish race condition in the buttons test suite.

Given there is some async wibox redraw operations, there is a tiny,
tiny chance the test suite callback will be executed before the
redraw. It has been seen on the CI, so it is not 100% impossible.

tests/test-awful-widget-button.lua

@@ -89,7 +89,7 @@ table.insert(steps, function()
 end)
 
 table.insert(steps, function()
-    assert(button._private.image ~= img)
+    if button._private.image == img then return end
 
     return true
 end)
@@ -115,7 +115,7 @@ table.insert(steps, function()
 end)
 
 table.insert(steps, function()
-assert(button._private.image ~= img)
+    if button._private.image == img then return end
 
     return true
 end)