Don't use a nonliteral format string in test/fheap.c:begin_test(). (#…

…1401)

* Avoid using a non-literal formatting string with sprintf in begin_test.
Use snprintf.  Don't duplicate a string on the heap unnecessarily.

* Do not deactivate -Wformat-nonliteral: we don't use a nonliteral format
string any more.

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