test: Ignore bottom panel in TestHistoryMetrics.testEvents pixel test

In the medium layout, the bottom panel sometimes has an extra white
background line (padding). This is random and unpredictable (probably
rounding noise?). That panel only contains a single standard button, so
is not very interesting.

Just ignore that part.

Fixes cockpit-project#21422

test/verify/check-metrics

@@ -247,7 +247,8 @@ class TestHistoryMetrics(testlib.MachineCase):
         b.wait_in_text("#metrics-hour-1597662000000.metrics-hour-compressed .spikes_count", "3 spikes")
         b.wait_in_text("#metrics-hour-1597662000000.metrics-hour-compressed .spikes_info", "1 Memory, 1 Disk I/O, 1 Network I/O")
 
-        b.assert_pixels(".metrics", "metrics-history-compressed-hour", ignore=[".nodata"],
+        # the bottom panel has some unpredictable padding height (± one pixel of extra background) in the medium layout
+        b.assert_pixels(".metrics", "metrics-history-compressed-hour", ignore=[".nodata", ".bottom-panel"],
                         skip_layouts=["mobile"], wait_after_layout_change=True)
 
         # Check that events are not visible for compressed hours