test(query-devtools/Devtools): add tests for resize handle (#10691)

* test(query-devtools/Devtools): add tests for resize handle

* test(query-devtools/Devtools): assert 'panel' is 'HTMLElement' instead of 'not.toBeNull' for stronger 'getBoundingClientRect' guarantee

* test(query-devtools/Devtools): assert non-null before parsing 'localStorage' value to prevent 'Number(null) === 0' false positive in decrease assertions