fix(tvix/nix-compat): require async and async,wire for some tests

Makes the following build:
  `cargo test --no-default-features --features async`
  `cargo test --no-default-features --features wire`

Change-Id: I47ba0c944f08895f67ed3b861706ef2e4ba384b3
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11739
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

tvix/nix-compat/src/wire/bytes/reader/mod.rs

@@ -414,6 +414,7 @@ mod tests {
     }
 
     /// Read the trailer immediately if there is no payload.
+    #[cfg(feature = "async")]
     #[tokio::test]
     async fn read_trailer_immediately() {
         use crate::nar::wire::PadPar;
@@ -431,6 +432,7 @@ mod tests {
     }
 
     /// Read the trailer even if we only read the exact payload size.
+    #[cfg(feature = "async")]
     #[tokio::test]
     async fn read_exact_trailer() {
         use crate::nar::wire::PadPar;