[pp-1] Convert DefaultMetatagsTest as Kernel test to use getNodeElementsByXpath()

Problem/Motivation

Split from #3583587: Convert some functional tests in system module to kernel tests. Once #3583594: Add getUrl() and (renamed) xpath() helpers to HttpKernelUiHelperTrait lands, we can convert core/modules/system/tests/src/Kernel/Page/DefaultMetatagsTest.php to use getNodeElementsByXpath()

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet