Make -Dtests.output=always actually work. (#648)

Signed-off-by: dblock <dblock@amazon.com>

TESTING.asciidoc

@@ -219,7 +219,7 @@ Print all the logging output from the test runs to the commandline
 even if tests are passing.
 
 ------------------------------
-./gradlew test -Dtests.output=always
+./gradlew test -Dtests.output=true
 ------------------------------
 
 Configure the heap size.

buildSrc/src/main/java/org/opensearch/gradle/OpenSearchTestBasePlugin.java

@@ -198,6 +198,7 @@ public class OpenSearchTestBasePlugin implements Plugin<Project> {
                 logging.setShowExceptions(true);
                 logging.setShowCauses(true);
                 logging.setExceptionFormat("full");
+                logging.setShowStandardStreams(Util.getBooleanProperty("tests.output", false));
             });
 
             if (OS.current().equals(OS.WINDOWS) && System.getProperty("tests.timeoutSuite") == null) {