`std.testing.fuzzInput`: introduce a length range option

Extracted from https://github.com/ziglang/zig/pull/20773.

```zig
test "fuzz example" {
    const input_bytes = std.testing.fuzzInput(.{
        .len_range = .{ .min = 8, .max = 8},
    });
    // perhaps we use this now to test 32-bit float printing
}
```