Extract tests in `test/libsolidity/ViewPureChecker.cpp` to `syntaxTests/viewPureChecker`

[r0qs]

As suggested by https://github.com/ethereum/solidity/pull/14759#discussion_r1461894555.

[AbhinavMir]

I'd love to hop on this! It'll be my first PR here, and had a question re: test running. If I make a new file in syntaxTests/viewPureChecker would it be read by the test runner automatically or do I have to add it in some file for it to be indexed?

[github-actions[bot]]

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