spec for gears.object: Test weak signals better

Since weak signals specifically exist to do magic with the garbage collector, do
a couple of garbage collection cycles and verify that this does not disconnect
the callback function prematurely (the function is kept alive since it is a
local variable).

(I actually did a mistake that makes this enhanced test fail and fixed it before
committing)

Signed-off-by: Uli Schlachter <psychon@znc.in>

spec/gears/object_spec.lua

@@ -52,6 +52,12 @@ describe("gears.object", function()
             called = true
         end
         obj:weak_connect_signal("signal", cb)
+
+        -- Check that the GC doesn't disconnect the signal
+        for i = 1, 10 do
+            collectgarbage("collect")
+        end
+
         obj:emit_signal("signal")
         assert.is_true(called)
     end)