Migrate Custom threshold > AVG - PCT - FIRED test to the deployment a…

…gnostic framework (#195902)

Part of #183378

## Summary
This PR moves the first Custom threshold rule test to the deployment
agnostic test. The rest will follow in a follow-up PR.

## How to run

To run serverless
```
node scripts/functional_tests_server --config x-pack/test/api_integration/deployment_agnostic/configs/serverless/oblt.serverless.config.ts
node scripts/functional_test_runner --config x-pack/test/api_integration/deployment_agnostic/configs/serverless/oblt.serverless.config.ts --grep="Custom Threshold rule"
```

To run stateful
```
node scripts/functional_tests_server --config x-pack/test/api_integration/deployment_agnostic/configs/stateful/oblt.stateful.config.ts
node scripts/functional_test_runner --config x-pack/test/api_integration/deployment_agnostic/configs/stateful/oblt.stateful.config.ts --grep="Custom Threshold rule"
```

### TODO

- [x] #195890
- [x] Test in MKI before merging


#### How to run tests on MKI

According to this
[discussion](elastic/observability-dev#3519 (comment)),
we should test in MKI environment before merging. For details on how to
run in MKI, see [this section of the
document](https://docs.google.com/document/d/1tiax7xoDYwFXYZjRTgVKkVMjN-SQzBWk4yn1JY6Z5UY/edit#heading=h.ece2z8p74izh)
and [this
readme](https://github.com/elastic/kibana/blob/main/x-pack/test_serverless/README.md#run-tests-on-mki).

x-pack/test/alerting_api_integration/observability/index.ts

@@ -10,7 +10,6 @@ export default function ({ loadTestFile }: any) {
   describe('Observability Rules', () => {
     describe('Rules Endpoints', () => {
       loadTestFile(require.resolve('./metric_threshold_rule'));
-      loadTestFile(require.resolve('./custom_threshold_rule/avg_pct_fired'));
       loadTestFile(require.resolve('./custom_threshold_rule/p99_pct_fired'));
       loadTestFile(require.resolve('./custom_threshold_rule/rate_bytes_fired'));
       loadTestFile(require.resolve('./custom_threshold_rule/avg_pct_no_data'));