[TFLite] Use structural checks in frontend tests

[tlopex]

This pr

• Replaces script-string based TFLite frontend IR checks with structural checks.
• Keeps numeric/runtime tests focused on VM output validation instead of mixing in structural assertions.
• Leaves the shared verify helper unchanged and use direct tvm.ir.assert_structural_equal where structural checks are needed.

[gemini-code-assist]

Code Review

This pull request refactors the TFLite frontend tests by replacing fragile string-based IR assertions with robust structural equality checks against expected TVM Script IR modules. Feedback on the changes highlights two main issues: first, in test_reverse_sequence, the structural assertion was accidentally removed, leaving the expected module unused; second, in test_unique, the runtime execution test was completely replaced by the structural check, and it is recommended to retain both to preserve runtime test coverage.

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