test: Drop mobile pixel tests in TestHistoryMetrics.testEvents

The mobile pixel test is way too flaky (> 50% failure rate on our CI).
scrollIntoViewIfNeeded() somehow misplaces the menu bar. This isn't a
real problem, an interactive Firefox/Chromium works fine.

Nobody has figured this out in two weeks, earlier attempts at fixing
that (commits e700960 and 790b377) also weren't effective. Figuring this
out is too much effort for too little benefit, and this flake breaks
pretty much every PR. So disable the mobile pixel test.

Fixes cockpit-project#20338

test/verify/check-metrics

@@ -238,16 +238,17 @@ class TestHistoryMetrics(testlib.MachineCase):
         b.wait_in_text("#metrics-hour-1597662000000:not(.metrics-hour-compressed) .metrics-events-hour-header-expanded .spikes_count", "3 spikes")
         b.wait_in_text("#metrics-hour-1597662000000:not(.metrics-hour-compressed) .metrics-events-hour-header-expanded .spikes_info", "1 Memory, 1 Disk I/O, 1 Network I/O")
 
+        # FIXME: mobile layout is racy in tests (only, not in reality), scrollIntoView() misplaces the menu bar
         b.assert_pixels(".metrics", "metrics-history-expanded-hour", ignore=[".spikes_count"],
-                        wait_after_layout_change=True)
+                        skip_layouts=["mobile"], wait_after_layout_change=True)
 
         b.click("#metrics-hour-1597662000000 button.metrics-events-expander")
         b.wait_in_text("#metrics-hour-1597662000000.metrics-hour-compressed", "1:00")
         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"],
-                        wait_after_layout_change=True)
+                        skip_layouts=["mobile"], wait_after_layout_change=True)
 
         # Check that events are not visible for compressed hours
         b.wait_not_present("#metrics-hour-1597662000000 div.metrics-minute[data-minute='28'] .metrics-events")