[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 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