Update compile fail test error messages

Tohe update to syn 2.x introduced some minor changes in
compilation error messages, most notably to #[construct_with]
which required adding a secondary error to provide the same
context-specific error message as before.

pnet_macros/tests/compile-fail/construct-with-errors.stderr

@@ -1,11 +1,17 @@
 error: #[construct_with] must have at least one argument
-  --> $DIR/construct-with-errors.rs:20:7
+  --> tests/compile-fail/construct-with-errors.rs:20:7
    |
 20 |     #[construct_with()] //~ ERROR #[construct_with] must have at least one argument
-   |       ^^^^^^^^^^^^^^
+   |       ^^^^^^^^^^^^^^^^
 
-error: #[construct_with] should be of the form #[construct_with(<types>)]
-  --> $DIR/construct-with-errors.rs:28:22
+error: unexpected literal in nested attribute, expected ident
+  --> tests/compile-fail/construct-with-errors.rs:28:22
    |
 28 |     #[construct_with("test")] //~ ERROR #[construct_with] should be of the form #[construct_with(<types>)]
    |                      ^^^^^^
+
+error: #[construct_with] should be of the form #[construct_with(<primitive types>)]
+  --> tests/compile-fail/construct-with-errors.rs:28:7
+   |
+28 |     #[construct_with("test")] //~ ERROR #[construct_with] should be of the form #[construct_with(<types>)]
+   |       ^^^^^^^^^^^^^^^^^^^^^^

pnet_macros/tests/compile-fail/length_expr_parentheses.stderr

@@ -5,3 +5,12 @@ error: this file contains an unclosed delimiter
    | ^^^^^^^^^ unclosed delimiter
    |
    = note: this error originates in the derive macro `::pnet_macros::Packet` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: proc-macro derive panicked
+  --> tests/compile-fail/length_expr_parentheses.rs:15:1
+   |
+15 | #[packet]
+   | ^^^^^^^^^
+   |
+   = help: message: compiler/fallback mismatch
+   = note: this error originates in the attribute macro `packet` (in Nightly builds, run with -Z macro-backtrace for more info)

pnet_macros/tests/compile-fail/variable_length_fields.stderr

@@ -1,5 +1,5 @@
 error: variable length field must have #[length = ""] or #[length_fn = ""] attribute
-  --> $DIR/variable_length_fields.rs:16:17
+  --> tests/compile-fail/variable_length_fields.rs:16:17
    |
 16 |     var_length: Vec<u8>, //~ ERROR: variable length field must have #[length = ""] or #[length_fn = ""] attribute
-   |                 ^^^
+   |                 ^^^^^^^