environment var moved:

1. SECP256K1_TEST_ITERS (replaces TEST_ITERS)
2. SECP256K1_BENCH_ITERS (replaces BENCH_ITERS)

This is a backport of secp256k1#1008
Depends on D19527

Backport note: we previously forgot to set the indirection to define SECP256K1_TEST_ITERS in the cmake build script. Now the cmake and autotools build will both use the same number of test iterations. The cmake build will generally be faster on CI, because unit tests will no longer default to 64 iterations.

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