Add tests that index with a const value.

In this commit tests were added to ensure that tests with a `const` index behaved as expected.
In order to minimize the changes to the test's corresponding `stderr`, the tests were appended to
the end of the file.

tests/ui/indexing_slicing.rs

@@ -68,4 +68,15 @@ fn main() {
     &v[..100];
 
     &v[..]; // Ok, should not produce stderr.
+
+    //
+    // Continue tests at end function to minimize the changes to this file's corresponding stderr.
+    //
+
+    const N: usize = 15; // Out of bounds
+    const M: usize = 3; // In bounds
+    x[N];
+    x[M]; // Ok, should not produce stderr.
+    v[N];
+    v[M];
 }

tests/ui/indexing_slicing.stderr

@@ -269,5 +269,27 @@ error: slicing may panic.
    |
    = help: Consider using `.get(..n)`or `.get_mut(..n)` instead
 
-error: aborting due to 38 previous errors
+error: const index is out of bounds
+  --> $DIR/indexing_slicing.rs:78:5
+   |
+78 |     x[N];
+   |     ^^^^
+
+error: indexing may panic.
+  --> $DIR/indexing_slicing.rs:80:5
+   |
+80 |     v[N];
+   |     ^^^^
+   |
+   = help: Consider using `.get(n)` or `.get_mut(n)` instead
+
+error: indexing may panic.
+  --> $DIR/indexing_slicing.rs:81:5
+   |
+81 |     v[M];
+   |     ^^^^
+   |
+   = help: Consider using `.get(n)` or `.get_mut(n)` instead
+
+error: aborting due to 41 previous errors