[SECP256K1] ctime_test: move context randomization test to the end
[SECP256K1] ctime_test: move context randomization test to the end

Summary:
```
I noticed this while reviewing ElementsProject/secp256k1-zkp#117 and
finding some seemingly unnecessary VALGRIND_MAKE_MEM_DEFINED that I
couldn't remove until I saw the bug.
```

Backport of [[https://github.com/bitcoin-core/secp256k1/pull/894 | secp256k1#894]]

Test Plan:
  ninja check-secp256k1

  libtool --mode=execute valgrind ./valgrind_ctime_test

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9389

Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc