JUnit Assume: Migrate to JUnit 5 Assume API

HelpWindowTest uses JUnit 4's Assume API to skip the test
in headless mode, due to a bug in JavaFX.
This bug is still yet to be fixed, hence we need to continue skipping
the test using JUnit's assumeFalse.

In JUnit 5, the API has migrated to org.junit.jupiter.api.Assume.*,
while the old APIs are left to maintain backward compatibility. While
the functionality remains largely similar, it is necessary to
standardize using JUnit 5 APIs as we migrate our tests to JUnit 5
style.

Let's migrate our calls to JUnit's Assume API to use the ones
provided by JUnit 5 instead.

In this migration, the API for Assume#assumeFalse(String, boolean) is
now changed to assumeFalse(boolean, String). The relevant method calls
have been updated to reflect this signature change.

src/test/java/seedu/address/ui/HelpWindowTest.java

@@ -1,9 +1,9 @@
 package seedu.address.ui;
 
-import static org.junit.Assume.assumeFalse;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertTrue;
+import static org.junit.jupiter.api.Assumptions.assumeFalse;
 import static seedu.address.ui.HelpWindow.USERGUIDE_FILE_PATH;
 
 import java.net.URL;
@@ -56,7 +56,7 @@ public void focus_helpWindowNotFocused_focused() throws Exception {
         // stage2.isFocused() returns true. However, as reported in the bug report,
         // both stage1.isFocused() and stage2.isFocused() returns true,
         // which fails the test.
-        assumeFalse("Test skipped in headless mode: Window focus behavior is buggy.", guiRobot.isHeadlessMode());
+        assumeFalse(guiRobot.isHeadlessMode(), "Test skipped in headless mode: Window focus behavior is buggy.");
         guiRobot.interact(helpWindow::show);
 
         // Focus on another stage to remove focus from the helpWindow