Add tests for E0762

src/test/ui/parser/lex-bad-char-literals-4.stderr

-error: unterminated character literal
+error[E0762]: unterminated character literal
   --> $DIR/lex-bad-char-literals-4.rs:4:5
    |
 LL |     '●
@@ -6,3 +6,4 @@ LL |     '●
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0762`.

src/test/ui/parser/lex-bad-char-literals-7.stderr

@@ -10,7 +10,7 @@ error: empty unicode escape (must have at least 1 hex digit)
 LL |     let _: char = '\u{}';
    |                    ^^^^
 
-error: unterminated character literal
+error[E0762]: unterminated character literal
   --> $DIR/lex-bad-char-literals-7.rs:11:13
    |
 LL |     let _ = ' hello // here's a comment
@@ -18,3 +18,4 @@ LL |     let _ = ' hello // here's a comment
 
 error: aborting due to 3 previous errors
 
+For more information about this error, try `rustc --explain E0762`.