Fix ui tests for latest rust stable (#6310)

Author: bkchr

primitives/api/test/tests/ui/mock_only_one_error_type.stderr

@@ -5,21 +5,26 @@ error: Error type can not change between runtime apis
    |         ^^^^
 
 error[E0277]: the trait bound `u32: std::convert::From<std::string::String>` is not satisfied
-  --> $DIR/mock_only_one_error_type.rs:15:1
-   |
-15 | / sp_api::mock_impl_runtime_apis! {
-16 | |     impl Api<Block> for MockApi {
-17 | |         type Error = u32;
-18 | |
-...  |
-26 | |     }
-27 | | }
-   | |_^ the trait `std::convert::From<std::string::String>` is not implemented for `u32`
-   |
-   = help: the following implementations were found:
-             <u32 as std::convert::From<bool>>
-             <u32 as std::convert::From<char>>
-             <u32 as std::convert::From<h2::frame::reason::Reason>>
-             <u32 as std::convert::From<h2::frame::reason::Reason>>
-           and 18 others
-   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+   --> $DIR/mock_only_one_error_type.rs:15:1
+    |
+15  | / sp_api::mock_impl_runtime_apis! {
+16  | |     impl Api<Block> for MockApi {
+17  | |         type Error = u32;
+18  | |
+...   |
+26  | |     }
+27  | | }
+    | |_^ the trait `std::convert::From<std::string::String>` is not implemented for `u32`
+    | 
+   ::: $WORKSPACE/primitives/api/src/lib.rs:347:35
+    |
+347 |       type Error: std::fmt::Debug + From<String>;
+    |                                     ------------ required by this bound in `sp_api_hidden_includes_DECL_RUNTIME_APIS::sp_api::ApiErrorExt`
+    |
+    = help: the following implementations were found:
+              <u32 as std::convert::From<bool>>
+              <u32 as std::convert::From<char>>
+              <u32 as std::convert::From<h2::frame::reason::Reason>>
+              <u32 as std::convert::From<h2::frame::reason::Reason>>
+            and 18 others
+    = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)