tests: Abort if malloc() fails during context cloning tests

Summary:
Found by the clang static analyzer.

This is the worst true positive that it found. I feel somewhat proud.

This is a backport of libsecp256k1 PR774

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D7596