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.

We can't use `ignore=` for that, as the difference is the snapshot
height, not the pixel contents. So instead, temporarily hide the whole
bottom panel for the snapshot.

Fixes #21422

test/verify/check-metrics

@@ -247,9 +247,14 @@ 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")
 
+        # HACK: the bottom panel has some unpredictable extra white line at the bottom, but is also uninteresting
+        b.eval_js("document.querySelector('.bottom-panel').style.display = 'none'")
+
         b.assert_pixels(".metrics", "metrics-history-compressed-hour", ignore=[".nodata"],
                         skip_layouts=["mobile"], wait_after_layout_change=True)
 
+        b.eval_js("document.querySelector('.bottom-panel').style.removeProperty('display')")
+
         # Check that events are not visible for compressed hours
         b.wait_not_present("#metrics-hour-1597662000000 div.metrics-minute[data-minute='28'] .metrics-events")
         b.click("#metrics-hour-1597662000000 button.metrics-events-expander")