Test that the IntelliJ settings keep the implementation classpath

The settings' tests wrote and read back the native binary settings but
never a classpath, the one list in them. The new test round-trips two
jars through IntelliJ's serializer.

Author: abashev

open-java-format-idea-plugin/src/test/java/com/palantir/javaformat/intellij/PalantirJavaFormatSettingsTest.java

@@ -19,7 +19,9 @@
 import com.intellij.openapi.util.JDOMUtil;
 import com.intellij.util.xmlb.XmlSerializer;
 import com.palantir.javaformat.intellij.PalantirJavaFormatSettings.StoredSettings;
+import java.net.URI;
 import java.nio.file.Path;
+import java.util.List;
 import java.util.Optional;
 import org.intellij.lang.annotations.Language;
 import org.junit.jupiter.api.Test;
@@ -60,6 +62,18 @@ void keepsTheNativeBinarySettings() throws Exception {
         assertThat(reloaded.getNativeImagePath()).hasValue(Path.of("/opt/open-java-format"));
     }
 
+    @Test
+    void keepsTheImplementationClassPath() throws Exception {
+        var state = new StoredSettings();
+        state.setImplementationClassPath(List.of("file:/opt/formatter.jar", "file:/opt/guava.jar"));
+
+        var reloaded = new PalantirJavaFormatSettings();
+        reloaded.loadState(deserialize(JDOMUtil.write(XmlSerializer.serialize(state))));
+
+        assertThat(reloaded.getImplementationClassPath())
+                .hasValue(List.of(URI.create("file:/opt/formatter.jar"), URI.create("file:/opt/guava.jar")));
+    }
+
     @Test
     void ignoresTheNativeImageTheGradlePluginUsedToWrite() throws Exception {
         // Gradle plugins up to 21.0.1 wrote this for openjavaformat.native.formatter=true. The native binary is a