SYN-3798: add last_run_status and last_run_at to /tests/ response

[bchhay-splunk]

Resolves #ISSUE_NUMBER

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Before the change?

• need: SYN-3798: add lastRunAt and lastRunStatus to GET /tests/ response syntheticsclient#15 and SYN-3798: add new filters syntheticsclient#16

After the change?

Pull request checklist

• Acceptance Tests have been updated, run (make testacc), and pasted in this PR (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)

Acceptance Test Output

Does this introduce a breaking change?

• Yes
• No

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[greatestusername-splunk]

Reviewing changes in synthetics client

[greatestusername-splunk]

https://github.com/splunk/syntheticsclient/releases/tag/v2.0.6
Should be released and usable for this stuff. Thank you for contributing!

[bchhay-splunk]

@greatestusername-splunk i pulled in the vendor changes for the newest tag. should be good to go now!

[greatestusername-splunk]

Passes acceptance test

terraform-provider-synthetics 12:11> make testacc
TF_ACC=1 go test $(go list ./... | grep -v 'vendor')  -timeout 120m   | sed '/X-Sf-Token/d'
?       github.com/splunk/terraform-provider-synthetics [no test files]
ok      github.com/splunk/terraform-provider-synthetics/synthetics      38.343s

[greatestusername-splunk]

LGTM

Only updates datasources and adds to structures