test, ci: Lower default iteration count to 16

[hebasto]

The number of test iterations in the CI remains the same.

Resolves https://github.com/bitcoin-core/secp256k1/issues/1561.

$ ./build/src/tests 
test count = 16
random seed = caafe3b5ec6fc0244ddc948206ce7d50
Skipping run_sha256_known_output_tests 1000000 (iteration count too low)
random run = cc89e190330dd604628bf529a04173e7
no problems found