Test function in Kotlin cannot find function in main module - gradle

Question

I have a standard (multi-)project set-up in Kotlin using gradle, where I have a /src/main/kotlin/ and a /src/test/kotlin/.
I have a function in my main module that I would like to test, but when i create a test function in the same named directory in the test module, the function I wish to test isn't found and is not callable.
For reference, the root build.gradle
ext.kotlin_version='1.3.61'
repositories {
mavenCentral()
jcenter()
}
dependencies {
classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
}
}
allprojects {
apply plugin: 'idea'
apply plugin: 'java'
apply plugin: 'kotlin'
version '1.0-SNAPSHOT'
repositories {
mavenCentral()
jcenter()
}
dependencies {
implementation "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
compileOnly 'org.projectlombok:lombok:1.18.8'
annotationProcessor 'org.projectlombok:lombok:1.18.8'
implementation "org.nd4j:nd4j-native-platform:1.0.0-beta5"
implementation "org.apache.commons:commons-lang3:3.9"
implementation "org.apache.commons:commons-math4"
implementation "io.ktor:ktor-server-core:1.2.6"
implementation "org.jetbrains.kotlin:kotlin-reflect:$kotlin_version"
testImplementation "org.jetbrains.kotlin:kotlin-test:$kotlin_version"
testImplementation "org.jetbrains.kotlin:kotlin-test-junit5:$kotlin_version"
testCompile group: 'junit', name: 'junit', version: '4.12'
}
sourceCompatibility = JavaVersion.VERSION_1_8
targetCompatibility = JavaVersion.VERSION_1_8
sourceSets {
main.java.srcDirs += 'src/main/kotlin/'
test.java.srcDirs += 'src/test/kotlin/'
}
compileKotlin {
kotlinOptions {
jvmTarget = '1.8'
apiVersion = '1.3'
languageVersion = '1.3'
}
}
}
and the sub-project build.gradle:
plugins {
id 'java'
id 'org.jetbrains.kotlin.jvm'
}
sourceSets {
main.java.srcDirs += 'src/main/kotlin/'
test.java.srcDirs += 'src/test/kotlin/'
}
repositories {
mavenCentral()
}
sourceCompatibility = 1.8
dependencies {
implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk8"
implementation "org.nd4j:nd4j-native-platform"
implementation "org.apache.commons:commons-lang3"
implementation "org.apache.commons:commons-math4"
}
compileKotlin {
kotlinOptions.jvmTarget = "1.8"
}
compileTestKotlin {
kotlinOptions.jvmTarget = "1.8"
}
Any help would be greatly appreciated