test: Fix TestMachinesDisks.testAddDiskPool race

Waiting for the defaultVol.iso volume to appear in the API isn't enough
-- it also needs to bubble trough libvirt-dbus and Redux. Wait until the
UI sees the volume before trying to select it in the "Add disk" dialog.
The dialog won't (and shouldn't) dynamically update the available
selections once its open.

test/check-machines-disks

@@ -917,6 +917,15 @@ class TestMachinesDisks(VirtualMachinesCase):
         m.upload([os.path.join(BOTS_DIR, "machine/cloud-init.iso")], "/var/lib/libvirt/images/defaultVol.iso")
         m.execute("virsh pool-refresh images")
         wait(lambda: "defaultVol.iso" in m.execute("virsh vol-list images"), delay=3)
+
+        # wait until the volume bubbled through the UI
+        b.go("/machines#/storages")
+        self.waitPoolRow("images")
+        self.togglePoolRow("images")
+        b.click("tr[data-row-id=pool-images-system] + tr li:contains('Storage volumes') button")
+        b.wait_in_text("#storage-pools-listing", "defaultVol.iso")
+        b.go("/machines#vm?name=subVmTest1&connection=system")
+
         dialog = self.VMAddDiskDialog(
             self,
             mode="use-existing",