[SECP256K1] make test count iteration configurable by environment variable
Summary:
You can set the test iteration count by command-line parameter but it is very hard to control the iteration count used by make check and even harder to control the count when this is run through Bitcoin Core's build system. Using an environment var solves both problems.
Backport of secp256k1#851
Test Plan:
SECP256K1_TEST_ITERS=16 ninja check-secp256k1
Should succeed but:
SECP256K1_TEST_ITERS=0 ninja check-secp256k1
Should fail with error An iteration count of 0 or less is not allowed.
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Differential Revision: https://reviews.bitcoinabc.org/D9375