Add ui test for #100197

Recover from mutable variable declaration where `mut` is placed before `let`

src/test/ui/parser/issue-100197-mut-let.fixed

@@ -0,0 +1,6 @@
+// run-rustfix
+
+fn main() {
+    let mut _x = 123;
+    //~^ ERROR invalid variable declaration
+}

src/test/ui/parser/issue-100197-mut-let.rs

@@ -0,0 +1,6 @@
+// run-rustfix
+
+fn main() {
+    mut let _x = 123;
+    //~^ ERROR invalid variable declaration
+}

src/test/ui/parser/issue-100197-mut-let.stderr

@@ -0,0 +1,8 @@
+error: invalid variable declaration
+  --> $DIR/issue-100197-mut-let.rs:4:5
+   |
+LL |     mut let _x = 123;
+   |     ^^^^^^^ help: switch the order of `mut` and `let`: `let mut`
+
+error: aborting due to previous error
+