improvements to random seed in src/tests.c

Summary:

• tests: add warning message when /dev/urandom fails

in case this code should ever be used as an example, a warning is a nice
way of helping ensure insecure keys are not generated

• fix bug in fread() failure check

the two middle arguments to fread() are easily confused, and cause the
checking of return value to fail incorrectly (and possibly succeed
incorrectly.)

• Fixup for C90 mixed declarations.

Reported-by: Jonas Nick <jonasd.nick@gmail.com>

This is a backport of secp256k1's PR516

Test Plan:

make check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4984