Add initial unit test

src/test.bach/test/java/module-info.java

@@ -1,5 +1,8 @@
-open /*test*/ module test.bach {
-  requires run.bach;
+open /*for testing*/ module test.bach {
+  requires run.bach; // module under test
+  requires static org.junit.platform.console; // for running tests
+  requires org.junit.jupiter;
+  requires org.junit.platform.launcher; // for writing tests
 
   provides java.util.spi.ToolProvider with
       test.bach.BachTests;

src/test.bach/test/java/test/bach/workflow/WorkflowTests.java

@@ -0,0 +1,21 @@
+package test.bach.workflow;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Nested;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.ValueSource;
+import run.bach.workflow.Structure.Launcher;
+
+class WorkflowTests {
+  @Nested
+  class StructureTests {
+    @Nested
+    class LauncherTests {
+      @ParameterizedTest
+      @ValueSource(strings = {"", "a", "=", "a=", "=a", "a=/", "a=a/", "a=/a"})
+      void illegalArgumentThrowsIllegalArgumentException(String s) {
+        Assertions.assertThrows(IllegalArgumentException.class, () -> Launcher.of(s));
+      }
+    }
+  }
+}