How to write conditional step with boolean parameter in jenkins scripted pipeline job?

You need to convert this environment variable (of type string) to boolean using toBoolean() function:

stage('test cond'){  
    if(env.BUILD_TESTING2.toBoolean()){  
        echo "Yes equal - running the stage"
    } else {
        echo "Not equal - skipping the stage"
    }
}

Better reference parameters by params instead of env, this way they have the correct object type. So use:

stage('test cond') {
    if(params.BUILD_TESTING2) {
        echo "Yes equal - running the stage"
    } else {
        echo "Not equal - skipping the stage"
    }
}