make the jni throwing runtime test actually throw (#675)

Author: ktoso

Samples/SwiftJavaExtractJNISampleApp/Sources/MySwiftLibrary/Throw.swift

@@ -15,5 +15,8 @@
 import SwiftJava
 
 public func throwString(input: String) throws -> String {
-  input
+  if input.isEmpty {
+    throw MySwiftError.swiftError
+  }
+  return input
 }

Samples/SwiftJavaExtractJNISampleApp/src/test/java/com/example/swift/ThrowTest.java

@@ -25,4 +25,13 @@ void throwString() throws Exception {
         String result = MySwiftLibrary.throwString("hey");
         assertEquals("hey", result);
     }
+
+    @Test
+    void throwStringActuallyThrows() {
+        Exception exception = assertThrows(Exception.class, () -> {
+            MySwiftLibrary.throwString("");
+        });
+        assertNotNull(exception.getMessage());
+        assertTrue(exception.getMessage().contains("swiftError"));
+    }
 }