Ignore warning on unused struct in test

    warning: struct `OptSourceNoBacktrace` is never constructed
      --> tests/test_option.rs:10:16
       |
    10 |     pub struct OptSourceNoBacktrace {
       |                ^^^^^^^^^^^^^^^^^^^^
       |
       = note: `#[warn(dead_code)]` on by default

    warning: struct `OptSourceAlwaysBacktrace` is never constructed
      --> tests/test_option.rs:17:16
       |
    17 |     pub struct OptSourceAlwaysBacktrace {
       |                ^^^^^^^^^^^^^^^^^^^^^^^^

    warning: struct `NoSourceOptBacktrace` is never constructed
      --> tests/test_option.rs:25:16
       |
    25 |     pub struct NoSourceOptBacktrace {
       |                ^^^^^^^^^^^^^^^^^^^^

    warning: struct `AlwaysSourceOptBacktrace` is never constructed
      --> tests/test_option.rs:32:16
       |
    32 |     pub struct AlwaysSourceOptBacktrace {
       |                ^^^^^^^^^^^^^^^^^^^^^^^^

    warning: struct `OptSourceOptBacktrace` is never constructed
      --> tests/test_option.rs:40:16
       |
    40 |     pub struct OptSourceOptBacktrace {
       |                ^^^^^^^^^^^^^^^^^^^^^

tests/test_option.rs

@@ -7,13 +7,15 @@ pub mod structs {
 
     #[derive(Error, Debug)]
     #[error("...")]
+    #[allow(dead_code)]
     pub struct OptSourceNoBacktrace {
         #[source]
         source: Option<anyhow::Error>,
     }
 
     #[derive(Error, Debug)]
     #[error("...")]
+    #[allow(dead_code)]
     pub struct OptSourceAlwaysBacktrace {
         #[source]
         source: Option<anyhow::Error>,
@@ -22,13 +24,15 @@ pub mod structs {
 
     #[derive(Error, Debug)]
     #[error("...")]
+    #[allow(dead_code)]
     pub struct NoSourceOptBacktrace {
         #[backtrace]
         backtrace: Option<Backtrace>,
     }
 
     #[derive(Error, Debug)]
     #[error("...")]
+    #[allow(dead_code)]
     pub struct AlwaysSourceOptBacktrace {
         source: anyhow::Error,
         #[backtrace]
@@ -37,6 +41,7 @@ pub mod structs {
 
     #[derive(Error, Debug)]
     #[error("...")]
+    #[allow(dead_code)]
     pub struct OptSourceOptBacktrace {
         #[source]
         source: Option<anyhow::Error>,