Integrate bigkey to system test

The following discussion from !985 (merged) should be addressed:

• @michal started a discussion: (+1 comment)

  Hmm, I do not see anything that would prevent this test from being run for a PKCS#11 build? I believe this is what the prereq.sh script was guarding against?

  Also, if bin/tests/system/rsabigexponent/bigkey.c is not built any more, it should just be removed from the tree.

  (I have not looked at this system test closely, so I may be confused.)