Update clippy tests

Author: petrochenkov

src/tools/clippy/tests/ui/crashes/ice-6255.rs

@@ -9,7 +9,7 @@ macro_rules! define_other_core {
 }
 
 fn main() {
-    core::panic!();
+    core::panic!(); //~ ERROR: `core` is ambiguous
 }
 
 define_other_core!();

src/tools/clippy/tests/ui/crashes/ice-6255.stderr

@@ -9,5 +9,25 @@ LL | define_other_core!();
    |
    = note: this error originates in the macro `define_other_core` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: aborting due to 1 previous error
+error[E0659]: `core` is ambiguous
+  --> tests/ui/crashes/ice-6255.rs:12:5
+   |
+LL |     core::panic!();
+   |     ^^^^ ambiguous name
+   |
+   = note: ambiguous because of a conflict between a macro-expanded name and a less macro-expanded name from outer scope during import or macro resolution
+   = note: `core` could refer to a built-in crate
+note: `core` could also refer to the crate imported here
+  --> tests/ui/crashes/ice-6255.rs:6:9
+   |
+LL |         extern crate std as core;
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^
+...
+LL | define_other_core!();
+   | -------------------- in this macro invocation
+   = help: use `crate::core` to refer to this crate unambiguously
+   = note: this error originates in the macro `define_other_core` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 2 previous errors
 
+For more information about this error, try `rustc --explain E0659`.