Use mapMulti instead of flatMap(Optional::stream) in a plugin test

error-prone's StreamFlatMapOptional warned about
.flatMap(Optional::stream) in the IntelliJ plugin's settings page test,
where findCheckBox walks the component tree for the first check box.
It now uses the replacement error-prone suggests,
.<JCheckBox>mapMulti(Optional::ifPresent), which builds no stream per
element; findFirst still stops at the first check box.

The three tests that call findCheckBox pass, as do the plugin's other
tests.

Author: abashev

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

@@ -108,7 +108,7 @@ private static Optional<JCheckBox> findCheckBox(Component root) {
         if (root instanceof Container container) {
             return Arrays.stream(container.getComponents())
                     .map(PalantirJavaFormatConfigurableTest::findCheckBox)
-                    .flatMap(Optional::stream)
+                    .<JCheckBox>mapMulti(Optional::ifPresent)
                     .findFirst();
         }
         return Optional.empty();