REGTESTS: converter: add a test for the arithmetic and bitwise converters

REGTESTS: converter: add a test for the arithmetic and bitwise converters

None of the integer converters was covered by a reg-test so far. This
test checks add, sub, mul, div, mod, neg, cpl, and, or, xor, not, bool,
odd, even, htonl and hex2i at once, using the "int()" fetch as an input
so that no server nor client data is needed.

The operand of add, sub and div is also passed as a variable so that
both the constant and the variable forms are covered, and the documented
behavior of a null divisor is verified for div() and mod().

Note that hex2i() currently fails on a non-hexadecimal input instead of
returning zero as documented, the test matches the implementation.