perhaps also move it to the test directory?

See #4159

@gmaxwell Done.

@laanwj Dang, double work :)

Well you also slimmed bignum.h down, so let's go with this one.

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Alternatively, we could replace the scriptnum tests by data-driven ones, and get rid of bignum.h entirely.

Having bignum in the tests directory is fine for now. At some point we could move to data-driven tests, but it seems low-priority as long as we need OpenSSL for other things.