chore: Move mock struct to test file

[rlaisqls]

It is more appropriate for SymTab and Sym structures to be separated into test files since they are only used in tests, not in general code. Additionally, adding the Mock prefix will clearly indicate to other developers that these structures are intended for testing purposes only, reducing the potential for confusion.

Therefore, I propose moving these two structures to test files and renaming them.

[CLAassistant]

All committers have signed the CLA.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}