Previously, the `TestStrategy` `NgElementStrategy` used in
`createCustomElement()` tests was created once and re-used in each test
(due to complications related to how `customElements.register()` works).
As a result, the `TestStrategy` instance's state (e.g. inputs) could be
polluted from previous tests and affect subsequent ones.
This commit ensures the strategy instance is reset before each test.
PR Close#36114
PR Close#37226
1202d17b4c