runtime: add comments for gcCPULimiter testcases

[jianfengtony]

add comments to explain some constant values used in testcases.

the testcases have some expects, it is defined by some formula or constants, and is hard to understand, this commits add some detail derivation to explain why the formula and constants is that.

[gopherbot]

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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