Revert error in verify_ulp bound calculation

Fixes the calculation of the ULP value for very small values near
normal_min

Adds a test specialized to the small-input case to catch similar errors
in the future.

Also fixes the values for FULLY_CONNECTED S1 generator unit tests.

Signed-off-by: Ian Tayler Lessa <ian.taylerlessa@arm.com>
Change-Id: I47f1bee68f6fabb69b434572603dbfe6442be554