test: deduplicate scalar and field constants for n-1 and p-1

[therohityadav]

Following up on feedback from #1819, this PR deduplicates the hardcoded constants for the group order minus one (n-1) and the field prime minus one (p-1) within src/tests.c.

Changes:

• Defined scalar_n_m1 and fe_p_m1 as global static constants at the top of the file.
• Replaced all hardcoded hex occurrences of these values in:
  • test_scalar_check_overflow
  • run_scalar_tests (HALF_TESTS)
  • scalar_cmov_test
  • scalar_cases and fe_cases arrays
  • Local scalar_minus_one and fe_minus_one definitions.
• Verified that make check passes all tests.

[real-or-random]

Concept ~0

I'm not convinced this really improves readability or maintainability

[therohityadav]

Concept ~0

I'm not convinced this really improves readability or maintainability

Thanks for the review @real-or-random.

I understand the concern about readability . having explicit values in tests is often preferred.

However, my idea here was to treat n-1 and p-1 as constants rather than arbitrary test vectors. By defining them in one place , we eliminate the risk of typos that could occur when these hex strings are repeated across the file. It ensures that every test checking these boundaries is mathematically guaranteed to be checking the same boundary.

[real-or-random]

I'm still not convinced that I want to spend time on this, but let me leave as a note here that a new test relevant to this PR has been added in b99a94c.