coverage: Tweak tests/coverage/assert-ne.rs

This test is intended to demonstrate that a particular macro-argument span
doesn't get lost during span-refinement, but it turns out that span-extraction
currently doesn't yield any MIR spans for this position.

This patch therefore tweaks the test to add a function call in that position,
so that it still remains relevant to span refinement.

Author: Zalathar

Diff for: tests/coverage/assert-ne.coverage

@@ -7,7 +7,7 @@
    LL|       |
    LL|      1|fn main() {
    LL|      1|    assert_ne!(
-   LL|      1|        Foo(5), // Make sure this expression's span isn't lost.
+   LL|      1|        black_box(Foo(5)), // Make sure this expression's span isn't lost.
    LL|      1|        if black_box(false) {
    LL|      0|            Foo(0) //
    LL|       |        } else {

Diff for: tests/coverage/assert-ne.rs

@@ -7,7 +7,7 @@ struct Foo(u32);
 
 fn main() {
     assert_ne!(
-        Foo(5), // Make sure this expression's span isn't lost.
+        black_box(Foo(5)), // Make sure this expression's span isn't lost.
         if black_box(false) {
             Foo(0) //
         } else {