coverage: Add a specific test for #[rustfmt::skip]

tests/coverage/rustfmt-skip.cov-map

@@ -0,0 +1,12 @@
+Function name: rustfmt_skip::main
+Raw bytes (24): 0x[01, 01, 00, 04, 01, 0a, 01, 00, 0a, 01, 02, 05, 00, 0d, 01, 03, 09, 00, 10, 01, 02, 01, 00, 02]
+Number of files: 1
+- file 0 => $DIR/rustfmt-skip.rs
+Number of expressions: 0
+Number of file 0 mappings: 4
+- Code(Counter(0)) at (prev + 10, 1) to (start + 0, 10)
+- Code(Counter(0)) at (prev + 2, 5) to (start + 0, 13)
+- Code(Counter(0)) at (prev + 3, 9) to (start + 0, 16)
+- Code(Counter(0)) at (prev + 2, 1) to (start + 0, 2)
+Highest counter ID seen: c0
+

tests/coverage/rustfmt-skip.coverage

@@ -0,0 +1,18 @@
+   LL|       |//@ edition: 2024
+   LL|       |
+   LL|       |// The presence of `#[rustfmt::skip]` on a function should not cause macros
+   LL|       |// within that function to mysteriously not be instrumented.
+   LL|       |//
+   LL|       |// This test detects problems that can occur when building an expansion tree
+   LL|       |// based on `ExpnData::parent` instead of `ExpnData::call_site`, for example.
+   LL|       |
+   LL|       |#[rustfmt::skip]
+   LL|      1|fn main() {
+   LL|       |    // Ensure a gap between the body start and the first statement.
+   LL|      1|    println!(
+   LL|       |        // Keep this on a separate line, to distinguish instrumentation of
+   LL|       |        // `println!` from instrumentation of its arguments.
+   LL|      1|        "hello"
+   LL|       |    );
+   LL|      1|}
+

tests/coverage/rustfmt-skip.rs

@@ -0,0 +1,17 @@
+//@ edition: 2024
+
+// The presence of `#[rustfmt::skip]` on a function should not cause macros
+// within that function to mysteriously not be instrumented.
+//
+// This test detects problems that can occur when building an expansion tree
+// based on `ExpnData::parent` instead of `ExpnData::call_site`, for example.
+
+#[rustfmt::skip]
+fn main() {
+    // Ensure a gap between the body start and the first statement.
+    println!(
+        // Keep this on a separate line, to distinguish instrumentation of
+        // `println!` from instrumentation of its arguments.
+        "hello"
+    );
+}