Fix temporary file creation for validation_tests

Summary:
This should hopefully fix the test in validation_tests.cpp
on Windows, where tmpfile() is unusable, at least on travis.

Note for future reference: tmpfile() support seems quite broken on
Windows and should be avoided.

Test Plan: src/test/test_bitcoin -t validation_tests

Reviewers: freetrader, sickpig, kyuupichan, #bitcoin_abc, awemany

Reviewed By: freetrader, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D199