Test that the unused_macros lint works correctly if rules are malformed

The unused_macro_rules lint had a bug where it would regard all rules of
a macro as unused if one rule were malformed. This bug doesn't exist
with the unused_macros lint. To ensure it doesn't appear in the future,
we add a test for it.

src/test/ui/lint/unused/unused-macros-malformed-rule.rs

@@ -0,0 +1,15 @@
+#![deny(unused_macros)]
+
+macro_rules! foo { //~ ERROR: unused macro definition
+    (v) => {};
+    () => 0; //~ ERROR: macro rhs must be delimited
+}
+
+macro_rules! bar {
+    (v) => {};
+    () => 0; //~ ERROR: macro rhs must be delimited
+}
+
+fn main() {
+    bar!(v);
+}

src/test/ui/lint/unused/unused-macros-malformed-rule.stderr

@@ -0,0 +1,26 @@
+error: macro rhs must be delimited
+  --> $DIR/unused-macros-malformed-rule.rs:5:11
+   |
+LL |     () => 0;
+   |           ^
+
+error: macro rhs must be delimited
+  --> $DIR/unused-macros-malformed-rule.rs:10:11
+   |
+LL |     () => 0;
+   |           ^
+
+error: unused macro definition: `foo`
+  --> $DIR/unused-macros-malformed-rule.rs:3:14
+   |
+LL | macro_rules! foo {
+   |              ^^^
+   |
+note: the lint level is defined here
+  --> $DIR/unused-macros-malformed-rule.rs:1:9
+   |
+LL | #![deny(unused_macros)]
+   |         ^^^^^^^^^^^^^
+
+error: aborting due to 3 previous errors
+