Why my Spock unit tests fail in Gradle build after upgrading to JDK 8?

I'm trying to compile my application source using JDK 8, Groovy 2.4.4, Spock 1.0-groovy-2.4, Gradle 1.11 (my firm hass custom plugins built on top of this)

gradle -version
Groovy:       1.8.6
Ant:          Apache Ant(TM) version 1.9.2 compiled on July 8 2013
Ivy:          2.2.0
JVM:          1.7.0_51 (Oracle Corporation 24.51-b03)
OS:           Windows 7 6.1 amd64

In my build environment, I need to retain my JAVA_HOME to Java 7 for other projects.

JAVA_HOME = C:\Program Files\Java\jdk1.7.0_51
JAVA8_HOME = C:\Program Files\Java\jdk1.8.0_60

Due to this, in my build.gradle, I explicitly tell Gradle to use JDK 8

tasks.withType(JavaCompile) {
    options.fork = true;
    options.forkOptions.executable = "${System.env.JAVA8_HOME}/bin/javac"
}

tasks.withType(Test) {
    executable = "${System.env.JAVA8_HOME}/bin/java"
}

compileJava{
    sourceCompatibility = 1.8
    targetCompatibility = 1.8
}

The source files compile successfully, however when it reached unit tests (written in Spock), it fails with the below error.

Caused by: java.lang.UnsupportedClassVersionError: org/model/SDA : Unsupported major.minor version 52.0

I confirmed that SDA.class is compiled using JDK 8.

javap -verbose SDA.class | findstr "major"
  major version: 52

Any reason why only during unit tests I get the above error? Does that mean unit test is being run on JDK 7?