Improve test coverage of RSA verification

[briansmith]

We should build the following test cases. Also, we may need to change the structure of check_modulus_and_exponent so that line-based coverage tools are useful in more precisely reporting what's tested.

# Test the range of accepted public key exponents: [3, 2^34), only odd values.

# e == 3, the minimum supported.
Digest = SHA256
Result = P

# e == 0
Digest = SHA256
Result = F

# e == 1
Digest = SHA256
Result = F

# e == 2
Digest = SHA256
Result = F

# e == 4

# e is the maximum 33-bit odd exponent.
Digest = SHA256
Result = P

# Smallest odd 34 bit value.
Digest = SHA256
Result = F

# e is 256 bits.
Digest = SHA256
Result = F

# e is 2048 bits.

# e is 8192 bits.

# Test the range of accepted public key moduli: 2048-8192 bits, only odd values.
[...]

[briansmith]

Also, empty e (zero bytes long), empty n (zero bytes long), signature has leading zero byte(s), signature ends in a zero byte.