manager: add basic UT for config manager

[xhebox]

What problem does this PR solve?

Issue Number: ref #11

Problem Summary: Prepare for testing namespace manager.

What is changed and how it works:

1. Removed getMul since it is overdesigned.
2. The returned value of del is removed.
3. Added a basic UT for config manager

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 (Don't forget to add the declarative for API)
• Has weirctl change
• Other user behavior changes

Release note

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None