Add a test for defmac crate

Author: qryxip

tests/defmac.rs

@@ -0,0 +1,9 @@
+use defmac::defmac; // 2018 edition style
+
+#[test]
+fn incr() {
+    let mut acc = 0;
+    defmac!(incr => acc += 1);
+    incr!();
+    assert_eq!(acc, 1);
+}