Test for constant-time flag leakage in BN_CTX
This commit adds a simple unit test to make sure that the constant-time flag does not "leak" among BN_CTX frames: - test_ctx_consttime_flag() initializes (and later frees before returning) a BN_CTX object, then it calls in sequence test_ctx_set_ct_flag() and test_ctx_check_ct_flag() using the same BN_CTX object. The process is run twice, once with a "normal" BN_CTX_new() object, then with a BN_CTX_secure_new() one. - test_ctx_set_ct_flag() starts a frame in the given BN_CTX and sets the BN_FLG_CONSTTIME flag on some of the BIGNUMs obtained from the frame before ending it. - test_ctx_check_ct_flag() then starts a new frame and gets a number of BIGNUMs from it. In absence of leaks, none of the BIGNUMs in the new frame should have BN_FLG_CONSTTIME set. In actual BN_CTX usage inside libcrypto the leak could happen at any depth level in the BN_CTX stack, with varying results depending on the patterns of sibling trees of nested function calls shari...
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