Add test case for rust-lang#86177 and rust-lang#85718

src/test/run-make-fulldeps/coverage-reports/expected_show_coverage.unused_mod.txt

@@ -0,0 +1,7 @@
+    1|       |#[path = "lib/unused_mod_helper.rs"]
+    2|       |mod unused_module;
+    3|       |
+    4|      1|fn main() {
+    5|      1|    println!("hello world!");
+    6|      1|}
+

src/test/run-make-fulldeps/coverage/lib/unused_mod_helper.rs

@@ -0,0 +1,3 @@
+pub fn never_called_function() {
+    println!("I am never called");
+}

src/test/run-make-fulldeps/coverage/unused_mod.rs

@@ -0,0 +1,6 @@
+#[path = "lib/unused_mod_helper.rs"]
+mod unused_module;
+
+fn main() {
+    println!("hello world!");
+}