`MDCBottomSheetController` had a few subtle issues when the content view controller
contained a scroll view.
When I fix these issues in a later PR in this stack, I want to
add application-hosted tests.
The `ios_runners()` function re-declares the test runner targets
every time it's invoked, which currently prevents us from declaring
separate logic and application-hosted test targets in a single BUILD
file.
This splits out the runners into targets in
`//components/testing/runners/BUILD` and replaces the `ios_runners()`
function with a constant `DEFAULT_IOS_RUNNER_TARGETS` which other
test targets can re-use.
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