test: add tests for correct ferris capitalization

Author: nik-rev

tests/ui/parser/ferris-static-mut.rs

@@ -0,0 +1,3 @@
+static mut 🦀: &str = "ferris!";//~ ERROR Ferris cannot be used as an identifier
+
+fn main() {}

tests/ui/parser/ferris-static-mut.stderr

@@ -0,0 +1,8 @@
+error: Ferris cannot be used as an identifier
+  --> $DIR/ferris-static-mut.rs:1:12
+   |
+LL | static mut 🦀: &str = "ferris!";
+   |            ^^ help: try using their name instead: `FERRIS`
+
+error: aborting due to 1 previous error
+

tests/ui/parser/ferris-struct.rs

@@ -0,0 +1,4 @@
+struct 🦀 {}//~ ERROR Ferris cannot be used as an identifier
+
+fn main() {}
+

tests/ui/parser/ferris-struct.stderr

@@ -0,0 +1,8 @@
+error: Ferris cannot be used as an identifier
+  --> $DIR/ferris-struct.rs:1:8
+   |
+LL | struct 🦀 {}
+   |        ^^ help: try using their name instead: `Ferris`
+
+error: aborting due to 1 previous error
+