[Synthetics] Add multispace monitor API test suite in private location (#226042)

This PR closes #225431 by running the multispace monitor API test suite
in both public and private locations.

Public location tests are still skipped on cloud, as they are expected
to fail.

How to Test
	1.	Run the following command:
```
TEST_CLOUD_HOST_NAME=console.qa.cld.elstc.co \
TEST_CLOUD=1 \
NODE_TLS_REJECT_UNAUTHORIZED=0 \
CLOUD_HOST="issue-serverless-uksvx-pr226042-e71759.kb.eu-west-1.aws.qa.elastic.cloud" \
CLOUD_PASS="42m8ZieD9Ld1s8lB0u99L2zg" \
CLOUD_USER="testing-internal" \
TEST_KIBANA_URL="https://${CLOUD_USER}:${CLOUD_PASS}@${CLOUD_HOST}" \
TEST_ES_URL="${TEST_KIBANA_URL}:443" \
node --no-warnings scripts/functional_test_runner \
  --exclude-tag skipCloud \
  --config x-pack/test/api_integration/deployment_agnostic/configs/serverless/oblt.synthetics.serverless.config.ts \
  --grep "LegacyAndMultiSpaceMonitorAPI"
```
2.	Expected Result:
	•	Only private location tests should run.
	•	All private location tests should pass.

Note:
If the deployment is recreated, you may need to update the values for
`CLOUD_HOST` and `CLOUD_PASS`.

Author: cesco-f