test: v2 sample test supports multiple test cases & internal endpoint

[Bobgy]

Description of your changes:
Changes:

• v2 sample test supports multiple test cases -- many samples need more than one set of tests, either testing both v1 and v2 compatible versions, or testing multiple set of arguments, as shown in test: more samples in v2 sample test #5431
• use internal endpoint for accessing KFP API server -- so that when errors happen, we can see the detailed error log (we preserve URLs as external links by passing in an extra parameter called external_host)

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[Bobgy]

/retest

[neuromage]

/lgtm

[Bobgy]

Thank you @neuromage!
I fixed the issues, would you want to confirm again?

[neuromage]

/lgtm

[Bobgy]

/approve

[google-oss-robot]

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