Add envconfig test helper for manipulating Env config via
callables. Create new common modifiers for non-admin config,
validator config and one for using different server port values.
Avoid custom overflow code; simply use 128-bit math to
maintain precision and return a saturated 64-bit value
as the final result.
Disallow use of negative values in the `fee_mult_max`
and `fee_div_max` fields. This change could potentially
cause submissions with negative values that would have
previously succeeded to now fail.
