Migrate remaining test files from *.js to *.ts

[mshustov]

I intentionally omitted the broad migration across the whole codebases since most of the plugins already rely on *.ts in the plugin codebase but keep functional tests in *.js.

One of the pain points working on #113950 was lots of problems in ./test and ./x-pack/test folders caused by the lack of typings.
There are 412 *.js files in ./test and ./x-pack/test folders and lots of places that misuse any (like it used to be es: any here)

kibana/x-pack/test/alerting_api_integration/spaces_only/tests/alerting/builtin_alert_types/es_query/create_test_data.ts

Line 19 in 3c8fa52

es: Client,

I’m wondering whether we should start working towards *.js files migration to *.ts. We've already been tracking the number of any in the public API. Similarly, we could start tracking the number of *.js files per team, which in turn, requires defining owners for all the functional tests.

It isn't a critical problem. But it affects DX when someone (usually the Kibana Platform team) has to fix issues in an unfamiliar domain. For example, logic in ./test and x-pack/test do not have tests, and the only way to investigate an FTR failure is to debug it in runtime that is slow by design.

cc @elastic/kibana-operations

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)

[pgayvallet]

AFAIK, all the .js files under Core's responsibilities have been migrated