xtask: increase timeout for fuzz_mesh :(

The thing that is, apparently, making this necessary, is the new
`BlockDef` evaluation upon construction. (Is `Arbitrary` generating
and throwing away `BlockDef`s? I don't know how to be sure.)

tools/xtask/src/xtask.rs

@@ -246,6 +246,8 @@ fn main() -> Result<(), ActionError> {
                     // that deterministically limits how much time is spent on one
                     // (just like the recursion limit currently does).
                     "fuzz_block_eval" => 15,
+                    // also does a bunch of block eval stuff
+                    "fuzz_mesh" => 30,
                     _ => 5,
                 };