backend: fix authenticator tests

[djshow832]

What problem does this PR solve?

Issue Number: close #295

Problem Summary:

• The result of the function getCfgCombinations is wrong: the slice of append(o, cfg) is reused and thus overwrites previous slices, so the test doesn't cover all the cases.
• The cfgOverriders are wrong. E.g. cfg.clientConfig.capability = defaultTestClientCapability & ^pnet.ClientPSMultiResults overwrites other capabilities. It should be cfg.clientConfig.capability &= ^pnet.ClientPSMultiResults to only change the current capability. It also doesn't cover all the cases.

What is changed and how it works:

• Fix getCfgCombinations.
• Fix cfgOverriders.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Notable changes

• Has configuration change
• Has HTTP API interfaces change
• Has tiproxyctl change
• Other user behavior changes

Release note

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None