cmake: restore C89 compatibility of CurlTests.c
I broke it in d1b5cf83 and 97de97da. Reported-by: Viktor Szakats Ref: https://github.com/curl/curl/commit/97de97daefc2ed084c91eff34af2426f2e55e134#commitcomment-33499044 Closes https://github.com/curl/curl/pull/3868
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