RT ticket #62101: as_int() returns NaN for inf

RT ticket #62101: as_int() returns NaN for inf

Math::BigFloat -> binf() -> as_int() returns NaN, but should return
inf. In other words, if $x is a Math::BigFloat, then $x -> as_int()
shall return the same as Math::BigInt -> new($x). The bug appears both
for +inf and -inf.

-lib/Math/BigFloat.pm: Add two lines to catch the cases when the input
 is +/-inf and NaN, respectively.

-t/bare_mbf.t: incremented test count by 3

-t/bigfltpm.inc: add 3 tests, for +inf, -inf, and NaN

-t/bigfltpm.t: incremented test count by 3

-t/sub_mbf.t: incremented test count by 3

-t/with_sub.t: incremented test count by 3