[dhcpv6] Re-enable test broken by float arithmetic

Re-enable tests that were disabled due to floating-point arithmetic
changes in the rust standard library. The behavior of floating-point
arithmetic went back to rounding vs truncating in
rust-lang/rust#96051, so it's safe to enable
these tests again.

A new bug has been filed to track the work to avoid doing
floating-point arithmetic. The complication is that `StepRng` is
behaving in unexpected ways and preventing migration of the existing
unit tests.

Fixed: 92923
Bug: 102199

Change-Id: I3372193a8d59f1bc0d816e9921056bb85fb290d4
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/643581
Reviewed-by: Jake Fried <[email protected]>
Reviewed-by: Ghanan Gowripalan <[email protected]>
Commit-Queue: Tony Gong <[email protected]>

src/connectivity/network/dhcpv6/core/src/client.rs

@@ -5290,7 +5290,6 @@ mod tests {
     // NOTE: All comparisons are done on millisecond, so this test is not affected by precision
     // loss from floating point arithmetic.
     #[test]
-    #[ignore]
     fn retransmission_timeout() {
         let mut rng = StepRng::new(std::u64::MAX / 2, 0);