Build failure on mingw-64

test.exe build error: undefined reference to sscanf (gcc 5.3.0 x86_64-w64-mingw32 with -flto enabled)

[https://gitweb.torproject.org/tor.git/tree/src/test/test_util.c#n4667]

test/test_util.c line 4667:  int n = sscanf(contents, "%lu\n%c", &pid, &c);

Need tor_sscanf

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