test: add JUnit 5 harness

src/test/java/dev/marcospaulo/canalhandia/SanityTest.java

@@ -0,0 +1,18 @@
+package dev.marcospaulo.canalhandia;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+/**
+ * Guards the test harness itself. The rest of the suite is written test-first, so
+ * a silently broken harness (surefire not discovering JUnit 5, wrong JDK) would
+ * read as "all tests pass" instead of "no tests ran". This one is here to fail loudly.
+ */
+class SanityTest {
+
+    @Test
+    void harnessRuns() {
+        assertEquals(2, 1 + 1);
+    }
+}