[SECP256K1] Fix ability to compile tests without -DVERIFY.
[SECP256K1] Fix ability to compile tests without -DVERIFY.

Summary:
```
It's important that the tests are also run without -DVERIFY due to
 the possibility that side-effects of a VERIFY_CHECK fix a bug that
 would otherwise be detected.

Use of the verify_check macro in tests isn't sufficient.
```

Backport of secp256k1 PR628:
https://github.com/bitcoin-core/secp256k1/pull/628

This will also fix the build with coverage for secp256k1.

Test Plan:
  ninja check-secp256k1-tests

On secp256k1 standalone:
  ../configure --enable-coverage # will undefine VERIFY
  make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5307

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