Add more tests for raw string literal

Author: tesuji

tests/ui/single_char_pattern.fixed

@@ -57,4 +57,7 @@ fn main() {
     // Raw string
     x.split('a');
     x.split('a');
+    x.split('a');
+    x.split('\'');
+    x.split('#');
 }

tests/ui/single_char_pattern.rs

@@ -57,4 +57,7 @@ fn main() {
     // Raw string
     x.split(r"a");
     x.split(r#"a"#);
+    x.split(r###"a"###);
+    x.split(r###"'"###);
+    x.split(r###"#"###);
 }

tests/ui/single_char_pattern.stderr

@@ -144,5 +144,23 @@ error: single-character string constant used as pattern
 LL |     x.split(r#"a"#);
    |             ^^^^^^ help: try using a char instead: `'a'`
 
-error: aborting due to 24 previous errors
+error: single-character string constant used as pattern
+  --> $DIR/single_char_pattern.rs:60:13
+   |
+LL |     x.split(r###"a"###);
+   |             ^^^^^^^^^^ help: try using a char instead: `'a'`
+
+error: single-character string constant used as pattern
+  --> $DIR/single_char_pattern.rs:61:13
+   |
+LL |     x.split(r###"'"###);
+   |             ^^^^^^^^^^ help: try using a char instead: `'/''`
+
+error: single-character string constant used as pattern
+  --> $DIR/single_char_pattern.rs:62:13
+   |
+LL |     x.split(r###"#"###);
+   |             ^^^^^^^^^^ help: try using a char instead: `'#'`
+
+error: aborting due to 27 previous errors