build.gradle

apply plugin: 'java'
apply plugin: 'eclipse'
ext {
    projVersion = '1.0'
}
repositories {
    mavenCentral()
}
dependencies {
    compile group: 'org.seleniumhq.selenium', name: 'selenium-java', version: '2.+'
    compile group: 'junit', name: 'junit', version: '4.+'
    compile group: 'org.slf4j', name: 'slf4j-api', version: '1.7.+'
    compile group: 'ch.qos.logback', name: 'logback-classic', version: '1.+'
}
task testCacheSuite(type: Test) {
    // task to run a single JUnit test suite
    systemProperty 'logback.configurationFile', 'logback.xml'
    include '**/SuiteOfTests.class'
    testReportDir = file("${reporting.baseDir}/SuiteOfTests")
    testResultsDir = file("${buildDir}/test-results/SuiteOfTests")
}

Logback.java

// see build.gradle for imports
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import ch.qos.logback.classic.LoggerContext;
import ch.qos.logback.core.util.StatusPrinter;
...
public abstract class Utilities {
    public static final Logger logger = LoggerFactory.getLogger( "AnyUniqueStringHere" );
...
    public static void printLoggerState() {
        // print internal state
        LoggerContext lc = (LoggerContext)LoggerFactory.getILoggerFactory();
        StatusPrinter.print(lc);
    }
...
}
public class Test extends Utilities {
    @Test
    public myTest {
        printLoggerState(); // to console
        logger.info("Hello World");
    }
    ...
}

logback.xml

<configuration>
  <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
    <!-- encoders are assigned the type
         ch.qos.logback.classic.encoder.PatternLayoutEncoder by default -->
    <encoder>
      <pattern>%-4relative [%thread] %-5level %logger{35} - %msg %n</pattern>
    </encoder>
  </appender>  
  <appender name="FILE" class="ch.qos.logback.core.FileAppender">
    <file>junit.log</file>
    <append>false</append>
    <!-- encoders are assigned the type
         ch.qos.logback.classic.encoder.PatternLayoutEncoder by default -->
    <encoder>
      <pattern>%-4r %-5level %logger{35}: %msg%n</pattern>
    </encoder>
  </appender>        
  <root level="INFO">
      <appender-ref ref="FILE" />
      <appender-ref ref="STDOUT" />
  </root> 
  <!-- We want error logging from this logger to go to an extra appender 
       It still inherits CONSOLE STDOUT from the root logger -->
  <logger name="junit" level="INFO">
      <appender-ref ref="STDOUT" />
  </logger>    
</configuration>

I think you forgot to attach the SuiteOfTests.class file?

👍