Add tests for lexical equality of expressions.

[secure-sw-dev-bot]

This issue was copied from checkedc/checkedc#178

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This implements tests for checkedc/checkedc-clang#107

As part of statically checking bounds declarations, we need to check symbolic equality of expressions and to lexicographically order expressions (for sorting commutative operands). This implements language-level tests of bounds declarations that indirectly cover lexical equality/inequality.

[secure-sw-dev-bot]

Comment from @dtarditi:

As part of statically checking bounds declarations, we need to check symbolic equality of expressions and to lexicographically order expressions (for sorting commutative operands). This implements language-level tests of bounds declarations that indirectly cover lexical equality/inequality.