[TESTBUG] waitForIdle after creating frame in JSpinnerMouseAndKeyPressTest.java

Call Robot.waitForIdle() after createUI and before getting the location of the JSpinner component in the test: javax/swing/JSpinner/4515999/JSpinnerMouseAndKeyPressTest.java.

This is needed to ensure all the events for creating and displaying the frame are processed before interacting with the components on the screen.