Functional tests for various payload sizes and output buffer sizes.
When the padding is bad or the plaintext is too large for the output
buffer, verify that function writes some outputs. This doesn't
validate that the implementation is time-constant, but it at least
validates that it doesn't just return early without outputting anything.
The main goal with these tests is to test the bug in question and
they are not meant to test the entire PKCS#1 v1.5 behaviour. To
achieve full test coverage, further test cases are needed.
